volume pedal? no no no..that doesn't solve your problem..u want to crank up your amp to make sure that tube sound shines even at low volume? actually its not new lah.. use power attenuator..one common product is THD Hot Plate.
How does a Hot PlateŽ work?
A THD Hot PlateŽ is a tuned network of capacitors, resistors and inductors which adjusts the overall EQ as the volume is turned down to compensate for the human ear's frequency response. Your ear perceives sounds differently at different volumes: the louder the sound, the more sensitive your ears are to highs and lows. As the volume drops, your ear becomes more sensitive to the mid-range, and less so for highs and lows. The Hot PlateŽ compensates for this, working like the "Loudness" switch on a hi-fi. The THD Hot PlateŽ is the first, and for now, the only attenuator that is frequency compensated.
here:
http://www.thdelectronics.com/products/hotplate.htm
now u can hear a cranked up amp at low volume...even less than 30% of pot volume on the amp 8)