My thoughts were a possible PSU issue as the M60 can have the same power supply resistor problem as has been discussed at length for the DSP5000, but that would likely affect everything.

If it is not the PSU it could either be an issue in the crossover or the treble amp. As it starts to work after a period of time it could either be related to heating or an electrical effect such as a capacitor charging.

Would you feel confident to remove the back and take a look and post some photos?

Lounge: DSP5200, 518
Snug: DSP33 x 5, C61R, HD621
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