[steveaudio]

A tube power amp has different characteristics is from a solid state amp which will influence its performance with vintage or new speakers. The limiting element in a tube amp is the output transformer. A high fidelity transformer tested by itself rolls off the very high end beginning about 8khz, and rolls of the low end beginning about 50 Hz. Install that same transformer in a High (20 dB) feedback power amp and the response flattens nicely from 20 Hz to 20 kHz. But that flatness comes at a price. The price is twofold. 1) the damping factor begins to tail up at the ends of the spectrum and 2) the power response turns down at the ends of the spectrum. Power response at the high end falls off because of losses in the transformer. This is generally not a big issue since rarely is full output needed between say, 5 to 20 kHz. The low end is a different story. Transformer saturation will cause third harmonic distortion at high power levels, and transformer inductance will cause loss that will be reflected -- not in reduced frequency response -- but a tailing up of the damping factor and reduced power response. And where do we need power the most? At the low end!
The loudspeakers of the tube era were optimized using tube amps because at the time that was all there were. Modern speakers are designed and tested with solid-state amps. Does that mean you have to use a tube amp with, say, an old Advent? No, but recognize that there will be a difference in sound because the amp characteristics are different. I leave it to the dedicated audiophile to find a way to optimize the performance of legacy speakers when using modern amps.