Well for those who may be interested, that new tube I was waiting for arrived a while ago and I've had no problems since then.
I'd suggest for anyone receiving one to fiddle with the switches on the base of the unit, even if you don't understand what they do they tend to change the sound quite a bit and your headphones may be better suited to one of the non-default settings. I believe they're for adjusting the gain, but frankly I couldn't relate what was stated in the manual to my headphone's specifications. Either way, they might help you out.