I suppose you already know this, but please note that the Ultra 550 has tubes in it! "Only" in the input stage, of course.
Let me encourage you to give that a try. I own both the 550 and the U70 (equivalent to the Ultravalve), which I use to drive a pair of Ohm 2000; not as harsh a load as what you have there, but still . Initially, I preferred the U70 without much hesitation. This summer, the heat made me reconsider! So for about three months, I ran the 550 and I can honestly say that I was not missing the U70 much at all; it went from being a heat-induced compromise to a tradeoff (more power is nice).
They are close, much closer than any solid state amp has a right to make it. In particular, smooth with no hint of harshness.
Frank's recommendation is a typical example of his honesty. Since you are already using a push-pull kt77 tube amp, it's probably the case that the Ultravalve would actually work as a straight replacement for it (also push-pull, also kt77 power tubes, etc.). Sounds like an easy sale! But the 550 should be the better instrument for this setup. It costs a bit more than the Ultravalve, but you still save money compared to using the kind of active crossover that is up to par with AVA gear (e.g. a nice Marchand tube crossover). Finally there is the issue of getting the amps to work well together. Frank's amps are very fast, the bass amp that you are using will probably be less so. The IIIa crosses over at 300 hz, which includes a LOT of musical content, so you'll need to make sure that that bass panel is getting a darned good amp as well.
One obvious experiment is to get the 550 and keep it as the bass amp if you decide to bi/triamp after all.
When you test the frequency response in your current setup, see what you get in the uppermost octave, 10-20 khz. That's where the problem with the low impedance is supposed to show up.
