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!
lcherepkai
What's the sensitivity of your loudspeakers and in dBs how loud is loud enough?
I have a few pairs of loudspeakers that are in use in various rooms.  The Meadowlark Audio Heron hot rods are a 4ohm load with a sensitivity of 90dB while the Bohlender Graebener 520's are a 4 ohm load rated at 88dB.  I have a pair of large DIY Legacy Focus clones (not really clones but similar in appearance using Eton drivers with Peerless 12" woofers).  The sensitivity and ohm are unknown but were significantly more sensitive than the Von Schweikert VR4's they replaced.  I'm picking up a pair of Triangle Stratos Lunas tomorrow with a sensitivity of 93dB. 

With regards to how loud is loud enough?  Oh, on occasion, I do like some music quite loud.  Can you really listen to Rage Against the Machine any other way?  Or the Red Hot Chile Peppers?  Though I usually listen to jazz, both classic and modern and chill music or house music.  These are all listened to at levels well above background but would not preclude conversation.  Also note that different speakers reproduce sound in different ways.  The BG's are dipole ribbon hybrids and close up, they sound like they aren't producing much sound but step back and the sound fills in and fills up the room.   Thanks.