Can you give more information?
1. So the sound like sizzling bacon comes out of the subwoofer? Not from the plate amplifier itself?
Yes, I believe when I pressed the cone, it was damped. I can double check when it starts up again.
2. Does it do this all the time while the amp is powered?
Not yet. (see 15)
3. Does it sound like a clean hiss, or does it have crackles and pops? Can you describe it in more detail?
I think the latter, link here:
4. Does the sound vary in volume randomly? Does it change in volume if you adjust the gain control?
Yes in the first, and I don't know yet about the second.
5. What effect do you hear if you change the power option between "always ON" and "ON only when a signal is detected"?
No difference.
6. If you play a signal into it, do you hear that as well?
Yes.
7. If so, does it sound clean and undistorted?
Yes, except it's *adding* the sizzling bacon sound.
8. If so, can you play it at full volume and it still sounds OK?
I'm too scared to try full volume, I wouldn't want to fry my ears or go past xmax. 
But, when it starts sizzling again I can try increasing the volume.
9. Tell us everything you know about the history of the amp.
Listed at 15.
10. You say there's a hum when you connect more than one input... what are you using to provide the input signals?
RCA from two different receivers.
11. Can you connect the sources in either order and the hum only appears when both are connected?
Yes.
12. Are the signal sources powered from the AC mains?
My receiver is powered from the house's AC, and so is my amp - if that's what you mean.
13. Can you try feeding one input from an isolated device (e.g. the headphone output of a cellphone) when another input is connected?
I don't have an adapter cable to take 3.5mm to RCA.
14. Does the heatsink of the amplifier get warm when you play a low-level signal through it for ten minutes?
No.
15. Are there any questions that I don't know to ask, where the answer could help us become more familiar with your setup, the history of the amplifier, the details of the problem, etc?
This is my third one. They all work for a while, and this one for the longest - I used auto-on instead of always-on, and that reduced the heat.
On my previous amps of this model, I used always-on because I thought the auto-on wasn't working. It turns out, it just has a much longer delay than another model of amp I used previously
A tech told me that this issue is caused by heat.
I forget if I said this in my OP, but the sizzling issue comes after being on for a little while right now. With experience from my other amps, it will get to the point where it's always, or almost always present.
Also, often it doesn't ever turn off now when using the auto setting. I assume that's from the damage, as it worked before. And when I notice it a few hours later it's playing a louder tone (though not, or not always sizzling.) It seems to be a single tone.
Please try to answer ALL of these questions. Any one of them may give us the important clue that helps us understand the problem.
