can you alter the gain of an amplifier?

I never use active preamps and prefer sources with built-in volume controls over passive preamps when possible.  I've had great success using cd players by Consonance, Theta, & OPPO directly in my amp(s) but find that sometimes, even with the volume full up, the sound level is just not loud enough.  With my big, solid state amps, I know I'm not out of power, just not enough gain (this was particularly problematic with an Aleph clone).  I was wondering if it's possible/feasible to alter the gain of an amp to achieve better results using a passive volume control or the on-board volume control in the source?  I won't be doing the work myself.  I just want to know if a decent repair/mod shop could make the alterations.  Thanks!