"It has worked very well for years run off of a JVC pre-amp using the
speaker wire connections."
Why did you run a low level output (pre-amp) into a high level (speaker
wire) input??? You should have run a continuously shielded cable from
the RCA pre-amp out to the RCA line-in on the sub. You can't enter the
speaker connectors, without exposing bare wire and violating the
shielding. This really worked?
"After working find for an hour or two, the sub starts to feed back with
a low, but loud hum."
For the purposes of troubleshooting, distinguishing between feed back
and hum is important. Feedback is a resonance problem, where any
particular output frequency loops back into the early amplification
stage, exhibiting runaway gain. Hum is a ground path anomaly, or even
an outright capacitor failure where AC leaks into the signal path.
Feedback is untamed and wild, where as hum is a dull, steady state
buzz.
Time, temperature, and malfunction are common bedfellows.
Absolutely first, I would suspect the splitter, and then the cables
(ground loop). Swap it/them out, for known good connections. If the
problem persists, go back to the old pre-amp and run the good cables
(with out the splitter), to the subs line-in input. Let it go for same
amount of time to rule out thermal failure/overtime in the sub system.
If all is good, than the splitter or mono-out jack or circuit of the the HT
receiver is suspect.
Always address the little stuff first. Ask yourself, what is different in
the signal/power path now than before. Regress to a state where
things still work, or determine that each component systems functions
independently.
Something could be heating up over time, and causing a hum or
feedback. If you could find a way to download a 50/60 HZ tone, you
would better understand what AC hum sounds like, to confirm or deny
what's actually happening on your system
What bothers me is how you ran the sub in the beginning. Sending the
low output to a high level input would require maximum sub gain, to
be heard. Running flat out is tough on the amp (If indeed you did).
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