Dramatic Visible? Power Issues -- What Next?

My system has developed a new problem. It strikes me as whole lot more complicated than it likely is, but any input from those more knowledgeable than I (not too hard) would be greatly appreciated. This one’s got me worried.

Start simple: after being on for anywhere between 1/2 hour to two hours, the system develops a loud hissing, much like a very strong tube rush.

Complicate slightly: the volume of this "rush" is inversely proportional to the volume setting at the preamp. Which is to say that above a certain (variable, but too loud for listening levels) volume setting, the "rush" disappears. At a zero volume setting, the "rush" is the loudest, which can be very loud.

Complicate further: the volume, consistency and pattern (yes, pattern) of this "rush" changes. Sometimes it's constant. Sometimes it pulses. Sometimes it builds to peaks and then snaps to silence, only to build again -- sometimes rhythmically, sometimes erratically. It's in both channels. Preamp is tubed, amp is solid state.

Complicate yet more: it's tied to the power system and draws quite a bit of power. Yesterday, at zero volume and no input, the rhythmic "rush" pattern caused the lights in my apartment to pulse along with the soundtrack. (There is a Richard Gray Power Block between the system and said lights). This is when it started to scare me a bit. If that’s not enough, the amp clips while powering down and has blown internal fuses twice in as many weeks.

Prognosis? Clearly, a problem. Something is storing and then leaking a whole lot more power than it ought to be someplace it definitely ought not to be. This problem has been happening (rarely and subtly at first, but recently far more dramatically and without fail) ever since I got the preamp, so that is my first suspect. I have another preamp that I intend to swap in before I try anything else. In the meantime, any thoughts or pointers would be greatly appreciated. Looks and sounds like there is a lot of errant power in there someplace -- and I’d hate to stick my finger in the wrong spot...