This thread rates high on the search engines when one searches for info about the HD75/150, so I wanted to contribute a tip here...
I recently moved, and given my music gear is in a larger room now, I've had the volume up a bit more lately on my HD75. I noticed a terrible rattle inside the amp when I played a particular note. I opened the amp up and was able to see the rattle was caused by the equalizer board vibrating like crazy. There was no sort of dampening there (plenty of soft epoxy on the caps on the main board, though, which I do appreciate), so I insulated the entirety of the EQ circuit, aside from the faces of the sliders, with hot glue. The board cannot possibly vibrate on the four legs which mount it to the chassis now.
It's a very simple fix--you only need to remove around 8 screws to remove the chassis from the cab, disconnect the two speakers, and you're ready to get to work. I don't know how long it takes the charge to dissipate from the caps, but you should be careful not to shock yourself around them. If you haven't done this sort of work before, apply the hot glue slowly and build it up over time. If you apply all the glue at once, you may find the glue running down into the sliders before drying there.
The whole job is a roughly 20 minute fix. I also unscrewed the FX loop/phones jacks and put rubber grommets around the inside part of the barrels before screwing them back down from the front because I thought that board looked like it could be susceptible to vibration as well. If you want to do this as well, you can get a box full of assorted size grommets from Amazon. If double-sided, they can be sliced down the middle with a razor blade to reduce the profile.
The idea of hot glue may sound sloppy to you, but it's more or less inert and a very commonly used means of preventing ugly resonances in gear subjected to vibrations. The rubberiness of it also makes it durable--it would take a lot of blasting away at max volume playing the fundamental note to break it down.
On a side note, after having the amp open, I was quite impressed with the build quality, particularly given the price. The heatsink and ventilation looks adequate, so if it fails on me, it shouldn't be the usual culprit of temperature shock.