I'll start off by saying that I don't consider myself to be any kind of audiophile or anything like that and therefore any comments here are just my own personal view and I'm sure that there are some people somewhere that would disagree with me.
That being said....
Quick, short answer - the PA2V2 has more bass boost then the Aria.
More detailed: The PA2V2 is a wonderful little amp for the price, it's small and very easily portable. Via a pair of rechargeable batteries it's just amazing how long you can go on listening before you need to recharge. Via it the music that I listen to out of my DAP seems more controlled and responsive, the bass boost of the amp is just enough to me to make it enjoyable without overpowering the rest of the audio frequency in the same way that EQing from within the DAP itself might do.
However the PA2V2 is designed for the easier to drive headphones - it can provide the power for my KSC75, MS1 and HD555 and can output them at easily loud enough levels to be enjoyed but if I try the DT880 then it's struggling to cope with them. It can do a very good job for saying how power hungry these headphones are but it's just not really what it's designed for.
The Aria on the other hand seems more detailed overall - as though it's more clinical with it's handling of the music that is being passed to it. To me it's more like listening to exactly what the artist/audio engineer (no need to get into a debate on who ends up deciding what the music should sound like) was really after. To my ears there's slightly less bass then when passed through the PA2V2 but this isn't to say that it's not there - on music where the low level bass is there then the amp passes it on without any trouble. For example I refer to a song by The Charlatans called "You're so Pretty - We're so Pretty" from the album "Wonderland", when the bass kicks in on this track then it
really does kick and listening to my DT880s from the Aria I find it as close to listening with an external amp as I've ever heard (and the DT880s are not known for rock type music due to not sounding out much bass) To me it just shows what music does have the bass already put in and which music is more a case of EQ or the equipment.
As for the power to drive headphones, the Aria has as much as is needed for whatever I've ever thrown at it. With the DT880s I generally listen about half way on the volume scale though with certain tracks that are dynamically recorded to be quieter then most 'pop' music I have been known to turn the volume to 3/4 for the odd song or 2 (not too many though - don't want to ruin my ears) With the HD555 or MS1 I don't go any higher then about 1/3 max but the powers there to drive the headphones louder if that's your thing - I've had a couple of friends try it out and they max out the volume to listen at which scares me a bit as I can't imagine how people can listen to those volumes without damaging themselves!
The Aria also has the added advantage of having crossfeed (although only the "light" version is implemented) which I do occasionally make use of. I know that some people find headphones too fatiguing but personally I've never had this problem - I've always adored music and find silence very hard to deal with and so nearly always have something to listen to no matter what time of day or night and headphones don't really fatigue me apart from I generally lower the volume as I go to make sure I don't waste my hearing too much. The crossfeed is a nice feature that I have used on some music and find that it does make things a bit easier on the hearing on some music. What I do like is that on music where channel seperation is intentionally added in this crossfeed does keep the channels on the particular side. It's more a case of things moving a bit more central: left sided sounds are still on the left but they move in a bit. For those that require more crossfeed though I don't think this'll be enough and some of my older Beatles recordings are still hard to listen to due to all the vocals coming out of the right channel while the music is mainly out of the left... but then again anything but dual mono playback would have trouble with that (IMO)
I will add the proviso that my listening is mainly done using the inbuilt DAC of the Aria and comparing it to the PA2V2 that way is a little tricky due to the PA2V2 being plugged into the Audigy2 ZS and so the problems of that source could come through. I will say that from my X5L into the Aria sounds pretty dreadful to be honest. There just doesn't seem to be any synergy to my ears between the two though I only listened once or twice to see what things are like. The sound just wasn't enjoyable whereas through the PA2V2 the sound is just plain fun. I do find that the Aria's DAC to be good to my ears, I can't really say it's a night and day difference between that and the Audigy2 to me as I quite like that card and can still listen to music played through it and my computer speakers without being annoyed by the sound or anything, it still sounds really good althought with the creative speakers that I have I end up having to alter the bass response of the sub on pretty much every album seperately as it always seems just a little low/loud/off each time. This could just be me though. For games however (which is why I got this card) it really is wonderful, as it also is for listening to DVD films via (through the 7.1 speaker system) which is nice. Not a totally high-end system but good enough for my bedroom.
To conclude I would say that I have no qualms in recommending either of these amps to anyone. Other then comparing them one to another I can't really compare them at all as I've nothing to compare them too - these are the only two headphone amps that I've ever had and I really enjoy them both (but how much of that is pyshcoanalytical due to the amount of money that I've spent on this hobby I can't really say.. all I know is I enjoy them) I find that the Aria is good for analytical listening for home use whilst the PA2V2 works well with my portable system for just plain fun (think that's down to the extra bass that is heard through it) - while out and about I don't think that I can really pay as much attention to what I'm listening to even though I still enjoy it immensly.
Hope I've not rambled on too much...
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Edit:
Just realised I could talk about how both amps match up when I play my portable CD player through them - there's something that both can take and see how they compare? Obviously they can only (again) match up to what the source is that's powering them but I can see how they do with it.