I used the amp with stock tubes (100+ hrs burn in) with an Earman Donald DAC (pretty neutral and clean DAC). Both headphones have band new OEM Sennheiser pads and original cables. Using a mixture of Tidal Hifi and FLAC files (16 and 24 bit). There’s a few examples of songs used below however my comparisons weren’t solely based on them alone. I’ve owned the 650 for 8 ish months and the 600 on and off for 3 ish months.
Bass: Massive Attack - Angel, 2Pac - Ambitionz Az a Ridah**
Sub bass has a similar presence in that it’s there but not very impactful. The underlying ‘thump… thump thump… thump’ in Angel feels the same on both headphones which kind of surprised me but the main difference is in the higher bass frequencies. Mid bass is more impactful on the 650 but I think it’s due to the elevation rather than speed or dynamics. The 600 is surprisingly good though especially compared to solid state where I always found it lacking. This elevation makes the 650 really enjoyable imo and it doesn’t meddle with the mids… much… and on that note…
Mids and Timbre: Gordon Lightfoot - If You Could Read my Mind, Joni Mitchell - Court and Spark, Jazz at the Pawnshop - Take 5, Black Sabbath - Children of the Grave
I’m combining these two because there isn’t much to say beyond how good they both perform. With a solid state amp I find the 600 a little shouty for my tastes but on this amp it sits just about perfect. I think it’s the added bass presence putting it more inline with the mids. The 600 also has a clarity advantage and I think it’s because the bass on the 650 creeps in a bit. These are nit picky things but are noticeable when comparing the two. With both timbre is spot on.
Treble: Rush - Tom Sawyer, Van Morrison - Moondance, Fleetwood Mac - Never Going Back Again
So there’s this peak on the 600 and I cheated and looked at a frequency response graph. Looks to be at 8k or 9k and some people will prefer this because I believe this is where the ‘veil’ is somewhat removed in comparison. Along with that peak comes a more apparent amount of detail like the shimmer and decay of cymbals is much more present while listening to Tom Sawyer or the flute and piano in Moondance sounding much clearer and forward can be really nice. With the 650 it is there and when you’re listening intently you can hear it all the same (additional volume helps). For me this lead to being able to listen to the 650 for longer without fatigue setting in.
Soundstage: Yosi Horikawa - Letter, Pink Floyd - One the Run, Lou Reed - Walk on the Wild Side, Louis Armstrong - St. James Infirmary
650 is wider but not by much imo and probably only significantly noticeable if your going back and forth and specifically listening for it. Basically I don’t think for myself this would be a factor I’d base my decision on when choosing one. On both I don’t notice the gaps in the soundstage as much as with solid state and my thinking here is the slower decay and/or distortion does well to fill these in. Depth is so close it’s almost not even worth mentioning and I went back and forth on which performs better but gun to my I’d choose the 650.
Imaging and Separation: Rush - Twilight Zone, Jazz at the Pawnshop - Take Five
The 600 feels superior when listening to both in quick succession but I think that is due to the clarity and more apparent detail it has compared to the 650. However once I settle in with the 650 alone for a listening session I don’t feel myself wanting for more. Though I will say the added clarity gives the 600 a better sense of 3D staging.
I’ll wrap this up by saying there is no wrong answer here and these Senns sound extraordinarily good paired with the BHC+SB. In the end I find the 600 performs better for my personal preferences which is kind of saying something (at least to me) because I’ll be honest I had a pretty strong bias for the 650 when I started this. It’s the first headphone that seriously wowed me and I chose it over the 600 on budget amps and probably still do. That said, I have this amp now so the 600 stays.
I have Mullard NOS tubes I’ll be using now so I may give an update on how they sound eventually too.
Thanks for reading and I hope this helps some folks. I had fun doing it 
