Solid State
power amps are designed to sound pretty much the same at any volume (before you take into account clipping).
To be able to get your tubes cooking, and still retain a low volume level, I'd do something along these lines;
Amp (speaker out) > dummy load/speaker simulator > power amp > speaker cabinet
If you were to just send your effects loop into the power amp, then you'd lose the tone of overdriven power amp tubes. However, if you were to do something like what I suggested, then you'd retain the tone of both the preamp and power amp, but at pretty much any volume level.
