If I may clarify a little, the unit full of tubes on top is the power amp. There's no pre amp in the unit, just thru a pair of volume pots to control output. I don't call it gain pot because tube amps always running full gain, either idle or full volume!
The extra black box with the Abyss logo is the power supply section, which converts 12V DC power input to 560V output to the tube circuit's B+, which would enable the 4pcs of KT88s to give you full power of 75WRMS constant, in push pull configuration.
Do be carefull when opening the power supply box when unit is running, the shock is very nasty indeed! I once burnt my fingers touching a tube amp's power supply and that one only supplied 400V on B+.