Just like the Rockport speakers (at least the Avior but also the Altair remembering having seen their challenging impedance curve on one of these pages), the Sasha is a difficult load effectively.
Not just the fact the impedance is so low is a challenge for amps but the fact that the impedance changes so dramatically is as much a challenge! There is only one way to counter this: sufficient damping factor in combination with current.
And whereas tube amps may have the latter, they normally don't have the prior. Except those tube amps that try really hard to behave as if they were ss, think of CAT JL2 and JL3, VTL siegfried but we are far from SET sound, they do have a tube sound but so do certain ss-designs...
The reason why probably the Nagra Classic works well is because they must have a decent damping factor although I can't find it on the Nagra website just now. Since you don't seem to be listenening to music that needs the last word in dynamic headroom (Mahler-type symphonies, large choral work, full scale organ, etc) their limited power is not much of a problem.
If you use tube amps with lesser power and current, as there are loads in the market place, they will all be able to produce a very nice sound but coloured by the fact they have problems controlling the speaker across the whole frequency range (and this is certainly not necessarily an issue with the bass range, even the midrange can suffer heavily from it whilst the bass range doesn't!) . According to listening preferences and acoustics this might cause an effect that is desired or experienced as pleasing. Watch out, I am speaking only of (certain models of) the Rockport and the Wilson speakers.
I once tested a Voxativ mono amp, a 25-watt 845 tube amp, with the Aviors and the sound was glorious, silky, sweet, real ... If I only listened to girl or boy with a guitar, it would have been my first choice but as soon as you needed more, it was a different story.
Don't know how much the Sasha2 has remedied the impedance challenge of the first Sasha ...