As promised, a few comments on SOHA+JISBOS vs SOHA2 (both with GE 6680 tubes).
I'm been listening to the SOHA+JISBOS at work for hours on end and the SOHA2 at home. Tonight I have them side by side, both fed off a simple diyMod (Sonicap Gen1 output) and listening using MarkL modded D5000 headphones. I chose these headphones because they're very detailed and the "brightest" I have to try to catch all the nuances and faults of my music. I listen to a lot of music from French R&B (Jenifer, Koxie, Kenza Farah) to Rap (WuTang, 2Pac, Immortal Technique, Eminem - non-radio stuff, that stuff sucks) to rock (Linkin Park, Korn, System of a Down) to pop/dance (Kelly Clarkson, No Doubt, and even threw on Britney Spears for good measure).
Both amps sound great and I really enjoy them both, but having them this close together and listening to the same song with both, the differences are clear.
As some of you you know, I've been dealing with some background noise issues on the SOHA2 prototypes (yes, it's actually on both and when I let the amps warm up for a good 30 minutes, the noise is almost non-existent but still just barely there - definitely not something I notice when listening to music, even through the quiet parts).
So on to sound comparison.
First off, I have to say the SOHA2 is a speed demon! The bass (where it's most noticeable) is sharp, hard-hitting and very precise. The SOHA+JISBOS is a little slower and "mellower".
Bass (continued): More prominent in SOHA2, just a very good quality punch or rumble (when the song has it) instead of what I hear in the SOHA+JOSBOS which is a more mellower drawn out bass line. Certain victory for SOHA2 for my music taste, and I love strong bass, I just don't want to be overwhelmed.
Mids : Again the mids on the SOHA2 come out to the front a little, more in-your-face than the SOHA+JISBOS. You might say that SOHA2 is more analytical in the mids and it's easiest for me to think of SOHA2 as a strong classical singer while the SOHA+JISBOS is a more of a sultry jazz voice. Depends on mood to which I would prefer at the time; I would usually go for SOHA2.
Highs : The highs seem more sweet in the SOHA+JISBOS. I would not say the SOHA2 is less detailed, it's not, but the top end does sound sharp (I am trying to avoid using the world "clear" because there is no muddying of the highs at all in the SOHA+JISBOS, the SOHA2 is just more "sharp", perhaps even leaning to shrill with some instruments).
Note separation: SOHA2, no question. I'm pretty much note-deaf and I hear the instruments with this amp, not just a generalization of which notes they produce. The only other amp I hear that with is the Beta22.
How everything works together : Both amps have their (as I learned only today) distinct sound signatures. I think both are really great amps and I am enjoying both; choosing one isn't something I would want to do.
I will not have a final answer to which is better/worse, nor will I write a "conclusion". I like both, I'm very likely to keep both and use both regularly.
I will leave this note though: for Sennheiser HD600 & HD650, the SOHA2 is a clear winner.