Tube Amps With Good Bass Qualities

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.
 

jwlaff OP

I believe one may get good bass from this tube poweramp "Music Reference RM-200 Mk.II" power amplifier with speakers that have low impedance’s, using the purpose made for the job 2ohm transformer speaker output tap.

But as with most tube poweramps the lower the output tap used, the less wattage is available, so this needs to be taken into consideration.
A classic match would be Wilsons they are very low in impedance 2ohms usually in the bass yet quite high in efficiency 98db! so could be a good match for the this amp on the 2ohm tap.

Cheers George

Couldn't one argue that the better set-up if a tube amp is preferred, would be speakers that have powered bass drivers with a tube amp for the mids/highs? Or as an alternative, bi-amping with either a SS or class D amp for the bass. This is not a reply to the OP but merely what came to mind while reading this thread which (thanks to all) I found very informative.