I was gonna say clean boost of choice too, but then Ehd suggested EQ. I like EQ a lot, and here is why:
As you lower the volume, the response curve from bass to treble really changes, especially at lower volumes. So, with the Master volume, using an EQ you can
1. Hit it harder while maintaining low volume and
2. Adjust B/M/T for changes due to the overall volume in frequency response
I think that is what the real issue. Not what the amp is doing, but what comes out of the speaker. So make some pre-adjustments to the EQ, along with some clean boost.