Tube Amps With Good Bass Qualities

 but the weight of the sustain doesn't match what a solid state leviathon can do.

This is so correct, all you have to do is play some sustained organ or similar to immediately hear the difference, and not just on subs, but on inefficient hard to drive floor standers too, and the winner is the one that tries to rearrange your innards.

Cheers George

all you have to do is play

Paula Cole -  Tiger

Sheesh. I can play that thing and it shakes the building. Easy. Just design the amp so it can't drain the power supply. Its not that hard- and its not rocket science.

Yeah- I can feel my guts moving, and the cuffs of my pants. It has nothing to do with tubes or transistors and everything to do with how well the amp can drive the speaker.

So how many tube watts does one need to drive a 3 way speaker with a flat frequency response rated at a minimum of 50W and can go down to about 33hz ?

everything to do with how well the amp can drive the speaker.

And a "leviathan" (as gavman stated) solid state amp will drive far more variety of differing speaker loads than tubes can, especially speakers with low impedance, and - phase angle, as this makes the low impedance much worse as seen by the amp. 

We had an "amp try out fest" one Sunday a while back at friends house that had a pair of  JBL Synthesis 1400 Array's,
https://www.stereophile.com/content/jbl-synthesis-1400-array-bg-loudspeaker-measurements
His mate came over also with his OTL 60watters, they ran out of steam abruptly on bass dynamics at anything slightly above normal/low volume they were nice up till that, but it wasn't nice when it did, and the amps were fine, adjusted to spec and tubes just 4 months old.