Warning I may speak poorly of a few headphones in this review, if you get offended, remember that this is just one man’s opinion.
 
INTRO
The srs-3100 system is a combo of the SRM-252S amp/driver and the L300 earspeaker/headphone. The 252S is very nice and compact. I have considered upgrading this amp … but apparently, a proper electrostatic amp will cost $3000+. So I thought screw that and just compared the 252S to a 353X. the 353X opens the sound up a tiny bit... but from a quick blind test I only got 3 out of 4 right for choosing the 353X and the difference wasn’t major enough and surely didn’t seem worth the $900 I would have to pay for this new amp so I thought nothing more of it. (cough might be saving up for a BSHE)

Comfort:
Stock is not great, there is an oval-shaped mesh dust cover on the inside of the earcups that has an edge that might dig into your ears when you wear them. However, I found that if you rotate this mesh 180 degrees this gives your ears more room and no more discomfort. This is because the mesh’s surrounding plastic is asymmetrical, one side comes in farther than the other. This fix actually makes the headphones quite comfortable. However, I wanted more.
I had some Brainwavz pleather earpads lying around, and I thought why not. I tried affixing these pads onto the earpad adapter thing but something just sounded wrong. Probably from too much venting as now there was nothing stopping air from escaping around the edges as the brainwavz weren’t square shaped. So I just used some tape and put the earpads directly onto the earcups without any adapters with a small straw at the bottom (inspired by the straw mod) and the comfort was much better and so was the sound IMHO. More on that later as the whole review will be based on the sound with these pads on.
 
One thing to note is that there is channel imbalance below the second notch on the dial, and balances out just before you reach the second notch. I find that if I were to listen to music I would rarely listen below this and typically listen at notch 3 or higher. Only in modern and highly compressed songs would I turn it down back to 2, which is still perfectly fine, as it is only when I go below this that there is a problem. Plus this issue is very common and present on just about every analog knob if you go low enough. 
 
now with the cons out of the way…
 
Soundstage, Imaging, and Presentation:
Soundstage being the size of the sound "stage", Imaging being how well and easy it is to pick out each individual thing and separate it from the rest, Presentation being the package of these two things. A headphone's presentation is very important to me, there are many "high-end" headphones that I just can't stand because of this. Some of which are the LCD-X & Elears, as their presentation feels congested to me or everything just feels it its fired directly into my skull (no proper out-of-head sound). The SR207 and Utopias come close. 
In my head, the hd800s are the kings of soundstage. …well at least in terms of size but the SR009 are around the same level, speaking strictly of soundstage size.
However, coming from an HD650 as my last favorite headphone, I prefer a smaller soundstage with great imaging and presence. And for these reasons, the 009s and 800 just sound disperse to me and lacks the presence and intimacy.
(intimacy, the feeling that you can reach out and touch whatever’s playing). This the L300 does very well.
Speaking of just having a large soundstage. In this department, I prefer speakers. I used to think that headphones could never pull off a convincing presentation of something actually in front of you. But now...
Enter "Out Of Your Head" (OOYH) by Darin Fong.
What is OOYH? Basically, it is the best Virtual Surround software I've ever heard, and you should definitely try it out. The thing unique about it is that it is simulated from taking Binaural recordings from REAL speakers.
My favorite preset being PBN Audio Sammy Speakers which to my ears sound have the greatest transparency and depth of soundstage. (transparency: the perception that sound is not coming from your headphones, has to do with clarity, transient response, etc.)
This Software makes everything sound like a binaural recording. which makes out-of-your-head-sounds extremely convincing (basically everything). 
When Listening to music I switch it on and off depending on the song. But I leave it on all the time for classical/acoustic music.
With this software, it makes the HD800 soundstage seem obsolete to my mind, as just about any headphone can now have amazing soundstage.
 
Highs:
Let me first say that I am very sensitive to highs, but I know them very well. I've been playing violin professionally and was part of an orchestra, so I know my string instrument timbres very well. 
In the string ensemble piece "Death and the maiden" by Schubert, with OOYH on and left right channels flipped (so that the violins are to the right and cellos are to the left), it is the single closest experience I have ever had to being in the center of the second violin section within the string ensemble without actually being there. If I were to close my eyes I can see the concertmaster (Roman Simovic in this case) giving it his all with sweat on his brow. The violins just sound so real so present. They are just so detailed, with such great clarity and transparency to produce what I illustrated above. However, the upper highs on the L300 might be a bit more in quantity than what might be considered "neutral" but I cannot find any fault with them for any acoustic instrument, as the slightly raised highs just come off as perhaps greater room ambiance. Only in synthetic music will it come off as a tad excessive, but keep in mind that I am sensitive to treble (which also scratches off basically all AKG and Beyers from my list).
Now compared to HD800, even though the HD800s have a greater quantity of highs. It is strange that in a side by side comparison, I find the hd800 to sound veiled. Possibly from the lack of detail compared to the L300. I was plenty surprised by this. (For those wondering the amp was an HDVD800).
 
Mids:
HD650 remains my favorite for mids, not to say there is anything wrong with the mids on the L300s, just that I feel that the mids in the 650 are a hint colored to enhance the human voice and I think I prefer this. But the Mids are more transparent on the L300, It does no wrong here. Previous Lambdas have had issues here by sounding "shouty" I've experienced this personally, especially with the 407, but the L300 sounds much smoother sounding and do not have this problem.
 
Bass:
Estats produce bass quite differently from other transducers. There is a distinct lack of pressure beating against your ear. Which some may call a lack of impact. While this may be true, I never really enjoyed listening to particularly impactful bass as I find it extremely fatiguing. I like my bass but I can live without it. With the Brainwavz pads, the Bass gets a nice boost extends a little further into the sub-bass and does not bleed into the mids at all. This boost really helps out with some modern music and is like adding a subwoofer to the room. True that there is a roll off at 50Hz, but this is not noticeable unless you are listening to a heavy synthetic bass genre like dubstep.
I find my only complaints are that I wish that there was no roll off simply because it is more aesthetically pleasing to see a flat line down to 20hz on frequency graphs. Now the Koss ESP950... its bass is the most *trigger warning* pathetic sounding bass I have ever heard … like a hamster fart ... ok not that bad but it flutters and chokes and has absolutely NO impact, Nada. 
 
As a Whole:
There is absolutely no blurring of notes, imaging is so good that you can just reach out with your mind and hear each instrument as a solo performance. The L300 are just so musical and well rounded. Everything it does, it does spectacularly. It's ability to reproduce the timbre of cellos, violins, the human voice is just fantastic, it does no wrong. With other headphones (non-estats especially) you have to take a leap of faith to believe there is an instrument in front of you. With these, they just appear out of thin air, and they just show you that they are right there in front of you.
While the L300 are not perfect, they are the closest I have had the pleasure of experiencing.
And that’s my review, It will be updated as I see fit, with new comparisons possibly.
 
Saw some people recommending songs in their reviews so.. this song sounds exceptional with these:
Billy Lockett - Wide Eyes 
(The first verse comes in and it's Just OH G***** ****)

The Grado SR80i presents everything very nicely, almost romantic, you can hear everything,
until you start comparing. Its not the most detailed, however it is very transparent. Its 
soundstage is not very large although initially it does seem so. Sounds that are supposed 
to sound far away, don't. e.g. a choir behind a main singer sounds about the same distance 
from you. However these are one of the best deals you can find used. Sometimes there are 
treble spikes, but they are rare, the bass is actually pretty nice, but does not extend  
fully.
Overall I really like these, is does almost everything very well, a great package, but they 
are not the best in any one area. So for that reason, I am selling these headphones.

*This is "review" is old, made a new one on the whole SRM3100 system here: https://www.head-fi.org/showcase/stax-srs-3100.22188/ *

(First off, let me say that this review is more of a LX00 vs X07 comparison with some extra details on the L300s,
I've listened for a few hours to all the Lambdas that have been released since 2010 and spent just under a month
with my L300s )

The L300s have an amazing soundstage that sometimes makes me question whether I left my speakers on, I
have not experienced this with any of my other headphones. These just present sound in such a natural
sounding way. The only negative I found is that the pads are thin, so your ears may touch the inside if the earpiece,
but this can be solved by using 207/507 (or 307,407 if you want non-black) pads, which I have done.
(Or just use thicker pads in general)

Jumping right to some comparisons,

HD800 compared to the L300 sounds veiled to me, now don't get me wrong, before I listened to the Lambdas, the HD800
was the pinnacle of transparency for me ...but it just isn't on the level of the Lambdas. (I hope I didn't open Pandora's box
by saying this...) they just aren't as transparent/natural sounding to my ears, oh and the HD700 sounds almost closed
in comparason.
Both the HD800 & 700 were powered by an HDVD800, with the same source used for the Lambdas.

HD650, while I loved the HD650s for its smooth and almost perfect mids, I can't get over its lack of extension in the
higher frequencies, making it lack the airiness I've come to love from Listening to STAX.

LX00s vs X07s:
the LX00s are warmer sounding than the X07s series, with less sibilance and a deeper textured bass
(a weightier, fuller tone, more sub-bass support?).the LX00s also seem less dependent on their seal.
X07s are more airy but also more sibilant, but things seem to improve with tubes,
bass kicks too strongly with edm, to the point that it almost hurts my ears (which i will call "bass spike"), it is the same kind of
high-frequency spike sound that I hear form some of my bass-light headphones when they play bass kicks
(I spent the least amount of time with the SR507s, so not sure if these generalizations apply)

Some of the gripes I have with the previous STAX lambdas were, especially with the 407,
female vocals feel too close, almost in your head kind of sound. Making things sound
somethings brash/harsh this might be attributed to the rise at 1khz in the frequency graph.
Also, the soundstage on the 207s feels too two-dimensional to me, lacking depth.

Recommended Song: Only Time - Enya
(there's just something special lambdas do to this song, that nothing I've heard can match)

Questions are greaty welcome!

Notes:

So LX00s vs X07s generally
LX00:
-nicer soundstage imho
-warm
-more bass
-weird swivel, less comfort
-vocals are farther
X07:
-bright
-more highs & airy
-better comfort

307
-female vocals closer, other sounds farther from vocals
-bass spike
-bass is deep but never the focus
L300
-vocals more delicate, more details?
-space seems larger?
-clearer?
-good bass, a lot less bass spike. maybe even a bass hump,
-ample thump
-less sizzle

BASS BASS BASS!!!
If you're looking for that open spacious "open-back headphone" sound but you aren't willing to give up the "closed-back" sounding bass, AND you're on a budget...  look no further, the ma500s are what you're looking for!
 
Design: The ma500 has a special design to it, as the drivers swivel independently from the ear-cup chassis. It is suspended in the centre of the earcup by what seems to be rubber. Anyways I believe the purpose for this is so that sound hits your ear at an angle, for a more natural sound. And rather than have a fixed angle like what many other headphones do, this one can be like a custom fit for anyone's ears.
 
Comfort: These are one of the most comfortable pair of headphones I have had the pleasure of listening to, I can wear them for hours! But due to the rubber surrounding the drivers, it does trap some of the heat from radiating away so they will get warmer than what you might expect something open-back to get.
 
Sound: Bass might be overpowering for some. It almost sounds like the Philips X2s (solely in the Bass aspect), except X2's bass was slightly more controlled and had more impact(kick?) while not muddying up the mid-range. Also compared to the hd650, they have very similar bass and sub bass, but the ma500 has a bit more of both, it is however not as natural sounding as the 650s. Keep in mind these are about a third of the price of the X2s and 650s. For the price, the ma500 is pretty good. The soundstage is about average for an open-back, the mids and highs as well. Basically the jewels of these are in the bass Department.
 
The Sony MDR-MA500 is great for kicking back with some bass heavy music/ EDM/ Ambient, but if you want to hear clear transparent vocals, these are probably not the headphones you're looking for.
Thanks for reading and happy listening!!!