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Troubleshooting a JE Labs 300b amp

Posted by ddelaney on July 10, 2017 at 08:49:55:
Many years ago, I built a pair of the JE Labs 300b monoblock amps. In the power supply, I omitted the first capacitor after the rectifier, so that the choke is the first component. It results in voltages at the 300b plate of about 340V, and about 82V and 230V at the 6SN7 plate stages.
One of the amps has exhibited, more recently, a noticeable drop in midrange and bass output. The other one sounds fine.
Measurements show that the voltages are nearly similar between the two amps, the resistors all measure what they should (no irregular drift from their original values). There are no crackling sounds.
Where should I look next to diagnose this problem? Do the capacitors (coupling and cathode) cause problems like this?
Thanks