Be sure that old can cap isn't in the circuit anymore!
With regards to the volume-problem, it might be because the signal coming from your digital unit isn't loud enough. That mono block is just a phase-inverter and a power amplifier. You'd might want to stick some kind of amplifying stage in front of that, to get a bit more voltage than the 6SL7 can deliver by itself. Be sure to plug in a big ol' power resistor, and measure the power output now and compare it to the results after.