Any advice on what to do to help prevent transformer hum? I believe the first thing to try would be a DC blocker type thing but have done a bit of research and these can possibly have a bit of a negative effect on power amps. Anything else I can try?

The amp in question is an Avondale ZAP 250 and when listening to music at anything other than very quiet the hum isn’t an issue, on the rare occasion though that I listen at quiet volumes for a bit of background music it’s annoying!