My current amp just blew me away when I tried it out in a music store. I actually didn't buy at first. I was in Birmingham, Alabama on business and stopped in one might after supper at a large music store (kind of like a local Guitar Center.) I looked at and tried out several tube amps. Then, I looked at the Peavey Vypyr, which comes in SS and Tube, all with the same SHARC processor front end.
The Tube 60 model had that tube presence that sets tubes apart from SS. It's a certain tonal quality that you learn to recognize. Later, when I got back home, I went to the local Peavey dealer and spent an afternoon playing with one and bought it. That was over two years ago and I still love the amp. The modeling front end is like having all the stompboxes and rack effects built into the amp instead of on the floor where you have to deal with cabling and setup and power supplies.
The tube section is one 12ax7 that serves as an dual inverter. Each half feeds a 6L6 in two phases, exactly like all tube amps ever built. One half of the signal goes to each tube and the output is combined in the output transformer, then sent to the speakers. In a standard all tube amp, the rest of the tubes are used for signal input stages, tremelo, reverb, tone circuits, and other necessary coupling functions. These are all 1-1 input and output unity gain devices. The meat of any tube amp is in the power amp stage. That's where you get the unique amp curve effect and saturation for "tube feel and response".
In every blind test I've read, the players couldn't tell whether the sounds were coming through tubes or SS amps when someone else was playing and they were listening.
This link talks about SS vs Tube amplification and how they differ.
Can solid-state sound really match that of tubes?
http://www.trueaudio.com/at_eetjlm.htm