What factors would cause an amp to play with little gain

I.E. the amp is set to modern (highest gain mode) and the gain knob is almost maxed and the volume is on like 3/4 before it is even audiable on any channel. every channel is very quiet, but channel 1 is quietest, channel 2 is in the middle, and channel 3 is loudest. all channels have no distortion (due to the lack of gain) and are rediculously quiet. what would cause this

I had a 2 channel DR that did this. It sounded fine other wise just didn't have the same amount of gain other rectos had unless dimed. And yet it still didn't have the right tone for the gain. I did swap preamp tubes first to see if this solved it, but in the long run had it looked at and a grid resistor was shot. Only cost about $60 and I had them give the rest of the amp a good once over.

Modern mode at 1 o'clock with the master cranked should be more than enough gain for any application.