Protection Kicking On At Low Volumes?

If the amps are a little older its not that weird for the protection relays to develop corrosion and become intermittent, and especially not that weird if two amps of the same brand and model develop the same relay problem.

The symptom would be intermittent operation at low volume, but OK at higher volumes.

If this is the case your preamp is off the hook.

The SP-4 uses 'transistor modules' and one of them may be putting out D.C. This would force your amp to shift operating points especially if its D.C. coupled at the input. Not a good thing. It's also possible that the power supply caps are bad in the SP-4 so that the rail voltages are not correct, or a voltage regulator has failed.

The preamp or source can be the problem. Run your tuner or something else for the same time or longer. If it still does it, then maybe it's the preamp. There may even be a new transmitter in your area that is getting in your system.

The RF (radio frequency) can be getting into the system somewhere. The carrier wouldn't be in the hearing range but still getting amplified, and your speakers may be trying to reproduce this RF, using a lot of power trying. Sometimes getting rid of it can be a problem. It can leak in through the interconnects, or somewhere else. I'm not saying this is the cause, just a possible one. Better shielded interconnects may help in some cases.

Not too long ago, I went past the water tower for our area. It has so many antennae on it, the tower was almost hard to see. Years ago, a small AM radio antenna was put up about two or three miles from my home. This came through our phones, and played music for the people we put on hold. For some reason, one line was louder the other. The music signal (not the RF carrier) was in the audible range. If it was digital, I may have had no idea it was there. I still have all kinds of home-brew RF traps in the wall jacks to block it, if that RF is still there.

Could be something environmental in the manufacturing process; perhaps the contacts were plated in a strong solution that was not rinsed well enough, could be humidity in your house. Let the tech fix it- it could be something other than the relay- but unless you want to pay the tech to go on a fishing expedition, play music and don't worry what it could be.

The problem may be DC coming from your preamp. Try another preamp to see if this solves problem or just power up the amp without any connections. Also, try a different set of speakers. There may be a speaker problem causing your amp to trip, the amps are only doing what they are supposed to do--protect themselves.