The tone will vary from type to type (manufacturer and time period/construction type).
Gain also varies. But most long black-plate versions will break up easier than their gray plate siblings.
I find there to be a quality of clarity that is associated with long-plate preamp tubes. The longer the plate, the greater the clarity/articulation.
Long-plate preamp tubes can also tend to be more microphonic than short-plate versions. It sometimes helps to move them down the line (from V1 to V2), or use an otherwise fine microphonic tube as the phase-inverter.
I have had good luck with microphonic long blackplates by using a tube damper,
and then if that doesnt work then I take it out of V2 (DR) and move it to a simpler amp like Princeton or Champ and they seem to be fine, or else as said above move it down the line in the DR....although I hate to IMO waste a nice blackplate that way....
I have owned 3 Rivera's, including an M60, and found them all to take to tube changes well. I was surprised how easily I could pick out the tonal differences in the pre's that were loaded up.