I built a stereo power amp, using two kitset 20W amps. For the front end I was using a mixer with an attenuator. This worked well. I decided to install a kitset (AliExpress) preamp. Now the output from the power amps has low volume and is distorted. I suspect this is due to an impedance mismatch. The trouble is I don't have a clue how to fix it. What sort of circuit do I need and do I put between the amps?