I've got some hiss going on with my late 70's HD 210 with phase inverter in the pre. It's been like this since I bought it a couple years ago. I thought recapping it might fix the problem, but having just put in all new quality caps the problem is still there. Also I cleaned all the pots with electrical/contact cleaner. The noise is less than it used to be, but still louder than my 65 watt 212 of the same era (also with phase inverter) which is so quiet I can record with it. The 210 is simply too noisy to record with at present, high end leaks into everything. I can get it out with plugins etc but it changes the sound too much to be worthwhile. Also it may be related to the reverb channel as it gets worse when reverb channel is on and turned up all the way. Yet even with the tank totally disconnected and reverb down or off there's still noise.
Also for whatever reason the power light flickers on and off sometimes. It doesn't effect the sound in any way, it's really more an annoyance than anything else. If someone's had this problem and resolved it I'm keen to hear about that as well.
I've got the amp open on the bench at the moment, so it would be great to get these issues fixed!