I'll do my best here to explain the sonic differences (at least from my experience).
I use the STA-6 in my main living room system, but that's not necessarily because I think the STA-6 is so much better than the Volt+D. It is better suited for that particular room, though. The STA-6 has a somewhat laid back presentation while still having a great bass. I use big waveguide speakers in my living room, and that combined with the fact that my big open living room/kitchen area has a lot of hard surfaces (lots of big windows, hardwood floors, kitchen countertops, etc.), makes it necessary for me to pair my speakers with an amp that is not too forward in the treble and upper midrange. The STA-6 is great for that purpose. I had used it for awhile in my upstairs media room, which has big couches, thick carpet with a thick pad underneath, and not many straight/hard corners, and the STA-6 sounds almost boring and lifeless in that room. In my livelier (sound wise) living room, however, it sounds amazing.
The Volt+D, on the other hand, has slightly better imaging and separation of instruments than the STA-6. I'd also say the sound is just a touch livelier than the STA-6, especially in the high frequencies, but it is by no means a "bright" amp. It still works fairly well in my main living room system when I've used it. The main and most immediately obvious difference is that it's lacking the bass slam that the STA-6 has. The bass is there, but it's not nearly as powerful.
All in all, I have found that STA-6 sounds like what most people who aren't really into tube amps think a tube amp sounds like: Laid back with a warm sound signature. I will say, though, that the bass is better than most tube amps I've heard.
However, the Volt+D sounds eerily similar to some low-powered tube amps I've heard, including a Magnavox console tube amp I used to have. Voices and midrange sound really, really good on the Volt+D. The high frequencies also have some good sparkle without being too intense.
I hope that helps a little.