Will Altec Model 19's benefit from lower powered tube amp?

You would need to move to an active crossover (mini DSP perhaps) to really take advantage of that.
How about bi-amping the woofer with solid state and a 45 SET or 2A3 SET on the mids and tweeters. Best of both worlds.
Not always. Depends on the amps you use for the bass. Recently did a trial at a friend's place. He has a BIG set of Westlake monitors and was biamping them. A high power Crown on the bass and tubes and high quality SS on the mid/treble. I thought the bass was a bit wooly or flabby. Rewired for full range with a single amp. The so called high quality amps are the current Ampzilla 2000 SE from Spread Spectrum. They are great sounding SS amps and the bass was noticeably cleaner and tighter running full range with the zilla's. If I needed big power these would be my amps. The truly are great sounding SS. But the message here is that any old SS on the bass may not be the greatest.

Sorry for diatribe,
I would caution that winding good 845 iron is extremely difficult, especially if you intend to produce solid low frequency output. It is far, far easier to wind a good 300B output transformer in comparison, and a 300B can make 18W in class A2 without a ton of difficulty (and you don't need that 1kV supply). The 3.3K/20W output transformer (300B) can also be a whole ton smaller than a 10K/30W output transformer (845).

If you're buying a commercial 845 amp, try to get one that has at least been tested to meet the published specifications. An 845 amp claiming 30W should be able to make that 30W at 50Hz/10% THD at the bare minimum, if not a little lower than 50Hz. The last $$$ commercial 845 amp I tested made over 40W at 1kHz/10% THD but only 2W at 35Hz/10% THD.
can you share which 845 amp it was that measure poorly ("only 2W at 35Hz/10% THD")?