The hum and distorted sound was from a power tube that red plated (ran too hot) shown in pic #1.
There is also a scorched a resistor (see close up in pic #3).
Estimate for essential work needed and options are as follows.
ESSENTIAL: replace power tubes and resistor $135.
OPTION #1: Also replace Chinese preamp tubes. Note: the bad power tubes are also Chinese, they're sometimes less reliable than other brands. Additional $45.
OPTION #2: replace speaker to improve the lacking bass response you mentioned. Will send separate email with recommendations. Feel free to call or txt with any questions you may have.