Help diagnosing issues with Kenwood KRC-503

I also noticed it doesn't happen if the tone controls are set at 50% or lower. If you turn up the treble it starts to happen when the treble control hits about 80%. It happens almost immediately if you turn up the bass control past 50%.

Something in the tone control? According to the block diagram all the signals go through that board before going on to the pre-amp or power amp.
I'm going to dig into that circuit a little.
O)r, you could have just wrote what the supply was.

When an amplifier IC like the 8215 fails, they often get hot very quickly. In less than a minute, you generally can' hold your finger on the back of the IC.

Is the voltage on either the acc or BU terminals of the radio dropping to less than about 10.5v?
Sorry about that, I was trying to avoid waking the kid running up and down the stairs. The garage is down 2 flights, and it was late.
Power supply is a TDK_Lambda JDS-150/12A. I believe it's a decent unit, and this hasn't happened when testing other head units.

The deck warms up as usual. Ironically the lights up front are the warmest part. The power amp is under the heat sink at the back, and that's getting warm, but not hot.

I will check the acc and BU terminals later today, and I sent an email for the repair manual.
Last night I also checked the voltages at IC2, the tone control IC. And they are all way lower than they should be. The manual says they should be 4.3V on 6 of the pins, I'm reading 1.5V with pulsing on pin 2 and 6 from about 1.3 to 2.2V.

Unforunately the probe on my scope seems to be going bad, so I might need to pick up a new one if you have any brand suggestions. The output on the scope was flickering last night if I wiggled the wires on the probe, this morning it's barely working at all. Ugh.
Yes. I tried auto. I couldn't find the instruction manual for the AWS 315P, but I found one for a found the instruction manual for the 620C which is from the same timeframe. I followed the initial setup instruction and don't get anything on the scope.

I did notice previously that I only got a dot on the scope when the sweep time was at the fastest setting. Maybe that area is the issue? I just don't have a ton of time to play with it right now, I have a big race in 2 weeks and I'm focused on getting the racecar done.
Don't throw the towel right now, a scope can seem dead if you touched the wrong setting. Allmost any time I start it after some time away from it, I think it broke down...
For the radio I make a guess: Some of the preamp stages are running on stabilized power. This supply has a problem and when you pull a few more miliamps it collapses. That may give you the bass/ tremble dependend on/off bump. So try to check these voltages first. It may be a dry cap or faulty regulator.
These old radios can be repaired, as they are "quite" simple.

Nothing like my problem... I have a drawer full of "Becker Mexico" radios, the best and most expensive ones you could buy in the 90's. They all worked well when the vintage cars (Porsche, Mercedes and Ferrari) went into storage. About 8 years later, when the cars were reactivated, none of them works any more. The EPROM containing the programming seems to have lost it's contend. All got the exactly same fault, Becker as a company is gone and no scope or soldering iron is going to fix this.
Vielen dank! I'm not throwing away the scope just yet, but I'm taking a break from it and going back to my DMM.

I checked some voltages on different ICs tonight, to try to get a feel for how the signal moves through the circuits. I will keep checking when I get time. I'm definitely finding some voltages that are around 1.5V when the spec says 4.3V, so now I need to work back and figure out where that is happening.