My HK AVR is too under powered to drive my RT9's. What I want to do is power them with a Class D power amp like a Fosi V3. Connect AVR to power amp with RCA' then wire RT9's direct from there. See any problems with this theory? Just looking for a cheap solution to power -up.

Unless the AVR has pre-outs that's not a good idea as multiple amplification degrades the sound and can not good for the speaker. The A9s should be easy to drive though, so I'm curious why that's a problem with your current AVR. What AVR are you using?

Check the stranded speaker wire connections at both the AVR and the speaker binding posts. It can happen if something gets moved or jostled.

A loose copper strand is enough to trigger an AVR's protective circuitry, but it will masquerade as a thermal shutdown when you turn up the volume.