90db 4 ohm with tubes?

I would say in general tube amplifiers respond favorably to higher load
impedance and potentially sound their best under these conditions.
However if the tube amplifier has good quality transformers, stiff power
supply and 4 ohm taps they can work well with 4 ohm speakers.

I heard the 4 ohm 89db Dynaudio Evidence Platinum driven by the Octave
push pull amplifier and the sound was just excellent.
I've heard the 4 ohm 91db Rockport Altair driven by the VAC Phi 300.1 (PP)
amps and also driven by the Absolare Passion PSET amp and these
produced much better sound than the MSB solid state amp on this fabulous
speaker.If your Cary amp has high caliber transformers and power supply
you'll be fine.

Regards,

I'd agree the problem here is a speaker with wide load impedance fluctuations coupled to a power amp with high output impedance. My stipulation of good transformer and power supply isn't the solution in this case.NFB is an option for lowering the amp's output impedance but IMO introduces more harm overall than good. Widely fluctuating speaker loads just seems like a bad design choice. Why not just design a speaker with a flatter impedance curve? This would allow more amplifier choice and flexibility.
Regards,

Bifwynne,
A good friend of mine recently bought a pair of the Rockport Altair speakers. They are a nominal 4 ohm load but are driven effortlessly by tube amplifiers and sound wonderful and very natural. For this reason I'll assume they must have a relatively smooth impedance curve and thus aren't restricted to high current SS amplifiers in order to be driven. I wish other designers would follow Andy Payor's example. If it's going to be a lower nominal lmpedance, strive to keep it reasonably flat.
Regards,