DC entering my amp?

I have an SPL Performer 800 amp that has a DC protection function. Once every week or two that function gets tripped, not necessarily while playing music. I use good quality cables and the connections are tight. I have a tube preamp ahead of the SPL so thinking that could be it. It is single-ended output only and the SPL is balanced only so I use the single-to-balanced interconnects from Benchmark Media. Source is a Lumin streamer and an outboard DAC. All components are plugged into an Audioquest Powerquest unit. Harbeth speakers connected to amp via good Kimber cables. I leave all components on all the time for the most part.

Any ideas on where/how to look for the problem? I did once just leave only the amp on for several days and it didn't trip.