My SR1a arrived 3 days ago, just in time for the Easter holidays. Initially, I have to say I was very underwhelmed, running them off my re-capped and fully restored Sansui AU-717 integrated (rated conservatively at 85WPC into 8 ohms, 100W seems to be the accepted standard). This is literally the first time my AU-717 has failed to step up to the mark. I had hoped that this amp would be up to the job, but everything written in this thread about the need for power (and good power at that), to make the SR1a shine is absolutely true. With the AU-717, the bass and p-r-a-t definitely suffered, grainy and distorted on big bass music at only moderate volume. Hints of greatness were there however, so Plan B: use Bryston 4B-SST-C as power amp with the Sansui on preamp duties....
Wow, just wow. Immediately, these were headphones transformed from basically a sloppy distorted mess into the most dynamic, transparent and responsive headphones I have ever heard. Bass is now very tight, impactful and distortion-free even at high volume. Massive Attack 'Angel' - verified. Bass heads still not need apply, but everyone else will be very very happy indeed.
Soundstage is easily the best I have heard. I had a set of HD800 on hand to A/B and width-wise they are comparable, but SR1a kills HD800 in soundstage depth, height and also imaging.
It was extremely useful to have HD800 on site for direct comparison. Not a fair comparison in truth, but SR1a makes HD800 sound muddy, never thought I would say that. With SR1a the presentation is just so 100% clear and 'unhindered'. HD800 by comparison struggles sometimes with some complex music and gives out a lot of 'resonance' or even 'boomy' (if I could call it that, can't find the right word) on bass heavy music. I hadn't noticed that before until I could directly compare. My live recordings now sound exactly as I heard them on the night, rendered perfectly. I do think if you demo these headphones you need to have your favourite cans side by side for comparison. You will be shocked to hear the difference.
And of course, amps really do make the difference. Bryston 4B-SST-C with 300W into 8 ohms completely transforms the SR1a. With the Sansui, everything was strained and pushed to the limit. Now, I have effortless dynamics and bass, with seemingly unlimited power reserve. Not exactly cheap, but I picked mine up used on Ebay for less money than most half-decent headphone amps sell for these days, and with 9 years warranty remaining! I've not had chance to listen to a Pass Labs but would love to compare.
Excited to hear
@llamaluv impressions re burn-in. Mine probably have less than 30 hours total, so hopefully even more room for improvement.