"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. I would
suspect, that the splitter cable is damaged, (and only a few strands are
are actually carrying the signal). Test it, or swap it out, for a known to
be good cable. If the hum persists, send the mono-out, with the good
cable, to another line-in input device. If all is good, than the problem
is in the plate amp somewhere.
Something is 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
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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