I have a Schiit Lyr here which makes some problems. It makes a quite loud hum. Sounds like a typical 50Hz hum. With headphones with high sensitivity it's annoying. But even with a HD-650 which is not very sensitiv it's hearable. Could it be the unregulated (my guess because I can see no voltage regulators) power supply?
Another more serious problem is the very loud turn-on and turn-off noise. I checked it with a DMM and there is a very high DC voltage at the output. It raises several volts, ~6V if I turn it on and ~3V if I turn it off. This is much too high and can damage the headphones. Why is there no relais which connects the phones after the amp has stabilized and discoonects them immediately at turn-off?