Just FYI, the TLE2426CLP rail splitters are EOL on Mouser, as are the OPA2134 op-amps. There are infinite numbers of both on eBay from China, but I doubt the quality is there, at least on the op-amps.
Probably a bad STK. Swap them side-to-side if you want an easy way to check. I have seen STKs do the following, all in the SX-780:
1 channel dead
1 channel distorted
Receiver won't come out of protection
Receiver blows main fuse
They can do all sorts of fun things.
The free section gets more traffic than the other sections, by an incredible amount. Free means they'll be gone ASAP, $5-10 may take a while. Your mileage may vary, but I have had a lot of success giving stuff away.
Put them together, list them on Craigslist in the free section, and someone will come by and pick them up. I have given away several boxes of similar LPs in the past. There's always someone who'll want them. It's easier than hauling them to Goodwill where they'll just get tossed or become...
The heat thing is wildly overrated. A normal Marantz 2245 shouldn't generate any relevant amount of heat during normal use, especially if it's been upgraded to LEDs.
To maximize your profit, sell the tubes separately to people who are into those, and then put normal boring 12AX7s in the equipment and sell it with those. If you have a receiver that's worth $1000 normally, and it has $1000 worth of fancy tubes in it, you're not going to get $2000 for it, or...
No, this is the worst possible receiver to start on. Get a little Pioneer, an SX-450, SX-434, SX-424, SX-535, that sort of thing, and start with that. The SX-9000 is super-complicated.