Despite the fevered claims of some tubaholics, there is a definite advantage to be had by using tubes above 50 Hz and solid stste below that point.
This certainly works well for me (presently) and has worked well in the past as well. I'm currently using my Quicksilver 300B SET amps to push Silverline Sonatinas in a near-field space of moderate volume. Bass without subs is OK, but certainly not visceral. Really good for a 9 watt SET amp compared to others I've heard. I've added an ACI Force XL sub (internal 250W SS amp). At first I found it a bit tricky to integrate, but once integrated the combination is very rewarding. I'd used an ACI Titan in the past (different space and speakers) and found it similarly musical, fast (able to keep up with very sensitive, fast horns), and rewarding. A good friend, who's already chimed in on this thread, tried bi-amping his Maggie 20.1's with a VTL 450 on top and various hi-powered SS amps at the bottom, eventually settling on Parasound JC1's. I got to hear a few different permutations while he was trying to decide on the SS amp, as well as having heard the speakers pushed only by the VTL's. Vertical biamping was a definite improvement in every way for those speakers (though ultimately I felt his room was a bit small for them). The tube/SS combination did work great to my ears. Later on he determined he was not happy with it and went to a second pair of VTL 450's on the bottom. Never got to hear that combo myself, but he liked it much better than the JC1 (ss) on the bottom. Anyway, it certainly can work well IMO.