I don't want to get too off topic, but I may as well finish what I started here. Sorry, tl;dr post incoming...
I spent the weekend listening to a lot of my headphones and I've pretty much confirm that the K1000 (pushed by LD MKVI) and GS1000 (EF-1) are my favorite. Both sound very hi-end to me. The K1000 is superior in technical ability, realism (too real sometimes), and are neutral, while the GS1000 is more musical and romantic (warm and cozy), with nearly as good detail and imaging (you may say it's exaggerated due to the way the drivers have been tuned, but it's what we hear). I may be one of the few here that think this, the GS1000 is superior to the RS1 all around. Cleaner throughout (lows, mids, highs), better detailing, with a nicer (subjective) sound stage. The RS1 are a bit too engaging for my liking. One thing for sure, I felt that smooth and warm tubes helped them out a lot.
As for tube rolling, I got all my 3 tubes. My mullard longplates doesn't sound good, I think the tube itself may be bad or that the sonic signature isn't for me. It's a bit congested and lacks detail compared to the other two. The brightness of the tube itself is very dim compared to the Brimar CV4003 I got, dunno if that is suppose to mean anything regarding SQ. The Amperex Bugle Boy is actually very nice with the GS1000. The overall sound is more lively and the midrange is more realistic than the CV4003. It's hard to say whether I prefer a relaxed sound or a more lively one. I've decided to try out one more tube that should fit in somewhere between, the Brimar 13D5. A lot of folks on the
Tube Asylum like the 13D5. The RCA cleartop just happens to be too detail focused for me. It's good with the ESW10, but not with the grados.
The sibilance issue I'm having appears to be caused by my source. The Stello DA100 has a warm, yet revealing sound trait that affects all my headphones with a variety of amps. You really can't say a pair a headphones or amp sucks if you don't look at your source as it is the root of the sound you get. The headphone is the instrument, the amp is the musician's state of being, the source is the musician's technical ability. I got a pico with dac on the way, looks like it'll be a few weeks before I get it though. I'll then be able to test out if source swapping does anything for me. The pico/dac to EF1 to ESW10 is going to be my work rig. I'm just messing around with the EF1 to see which headphones I like... I like them all. I plan on getting two more flagship model headphones to try out this year. A2000X and Ah-D7000 are likely contenders.
I'm pretty content with the sound I'm getting from the DA100 and I feel that the sibilance is a part of the recording. Some songs will be poor while others will be fine. I hear sibilance with my HT setup as well, just never really paid much attention. I'd probably have to drop 3-5K for a source upgrade to hear a significant upgrade in SQ and the sibilance would probably still be there. I figure I'd try out electrostats instead and then upgrade my source later. A vinyl setup is also a possibility, but that would take lot of work.
I feel that the EF1 edges out every other sub $400 amp I have with low impedance phones. X-can V3, LD MK III, LD MK I+. I say edges, as in small improvements here and there that I prefer with my ears. It's not like going from a $300 amp to a $1000 amp. The bonus with the EF1 is that it's much easier to tube roll as you only need 1 tube (no need for matching pairs) and the 12AU7 are widely available.
I'll try out opamp rolling once a thread gets started on it so I know where to go. I also plan on DIY'ing a Millet minimax to compare to the EF1. Another good amp to compare the EF1 with is the Gilmore lite (price wise). SS vs hybrid vs hybrid... As I mentioned before, I feel that that tubes are very good to have with grados. SS might be too detailed/sharp for me. Tubes can provide you a smoother/warmer presentation with some really lush vocals to die for.
As for the HD600/HD650... I don't want to put this in rating terms, but it's quicker this way. On a lower end setup, I feel that the HD600/HD650 are 6-7/10. They sound good, but I can't help but feel that they are lacking. Sluggish pace and a somewhat sloppy presentation. In my case, I upgraded to an MK VI with 5998 and CV1985 tubes, and recabled the senns. About $1500 worth of upgrades to push the senns to 9/10 levels. Very nice instrumental separation and I get a better sense of realism. I can finally hear the vocals really clearly and I get a lot more detail throughout.
On the other hand, low impedance headphones on a more budget conscious setup will sound like 8/10. The RS1 and GS1000 don't sound like they are lacking much except the more realistic presentation I get from pushing the senns. The tough question for me is whether or not low-impedance phones will scale just as well when driving them with a higher end amp. I actually never had any intention of trying Grados because I felt happy with my Senns. I only got them because I could get them for 40% off MSRP, and it turns out that I think more highly of the grados.
I wrote the last two paragraphs because I believe the Senns deserve a more powerful amp to be appreciated. The EF1 and other sub $400 amps will work fine and are more reasonable when you factor in price. I don't think I'm getting 5x the SQ performance paying 5X the cost. More like 2x. I'm just not that impressed with the Senns on a more budget friendly rig.
My MK VI has two gain settings. One for K1000, the other for HD650, K701. I tried the HD650 with K1000 gain and I got some bad results with feedback(?) or noise. It sounded a bit thick and dull in a way. If this happens with low impedance phones on the normal gain, then I don't think the amp would be suitable so I'm stuck here. I never have the volume past 15-20/100 in any case. The K1000 on high-gain or HD650 on low gain. The WA5 looks like a good option to test hi vs low impedance scaling, but then the senns won't be balanced. Tough...
