After going over some tests with Alvin, I have noticed that no matter what combination of outputs I use from the preamp(both sets of XLR outputs, both L/R channels) as long as the left channel of the amp is involved in the mix, it goes into protect when using the preamp. Does not do this coming straight in from the DAC.Clearly the preamp is making a bit of DC at its output. It may well be that your powered speakers have coupling caps at their inputs. At any rate, the preamp needs service. It likely has 4 output coupling caps as its running balanced and likely also does not support the balanced standard or else it would have output transformers.
Any competent service technician should be easily able to sort this out.