Lower Volume from One Monoblock

Have you tried reversing the interconnect cables? Try swapping the cables from the preamp to the mono amps to eliminate the possibility of the preamp causing this problem. Also, are your cables of the same length and same kind? I had a similar issue as you, until I used the same length interconnect.

Good luck,
Audioquest4life

Make sure the 2 switches for Balanced/SE operation are both in the same position. If you have one on SE and the other on Balanced, the volume for each channel will be different. After that, swap the IC's going from your source to the preamp. Its possible that one of the other components is the problem.

The Bal/SE switches should be checked, however, interconnects and other components will not be the problem. Since the reduced sound moves as the amp is moved, it can only be the amp.

After reading Tls49's response, I think I may have missed something.

"Swapping the monoblocks left to right the louder sound swapped too. The left channel is now lower in volume."

Did you actually swap the amps so that the IC's going from the preamp to amp remained connected to the same channel, or did you just swap the IC's? If its the former, then Tls49 is right. Disregard everything in my 1st post except the part about the Balanced/SE switches.