tube amp/flutter in speakers?

I have no idea what it might be. Keep in mind that some speakers are capable of going MUCH lower in frequency response than others are. I have one set of speakers that will throw madly at 5 Hz while others do not flicker in the least. As such, your ESS's with their passive radiator loading may be revealing a problem that the other speakers simply can't reveal. Either way, i would not chance damaging ANY of your equipment without first having them checked out.

Glen, i've seen flame-out's at several different shows. The most significant one that i remember was at one of my buddie's shows. His bass amp ( Gallien Krueger ) started leaking DC and the speakers began smoking and then literally spit out flames. People thought it was like Ace Frehley's old "smoking Les Paul" routine. The reason that i remember this one so vividly though is that they were MY bass cabinets that he ignited !!! Needless to say, he replaced the drivers with brand new ones. I didn't come out of that one too bad, as i ended up with 4 brand new 18" drivers that cost about $180 apiece : ) Sean
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Kona, I had the exact same sympom from one of my monoblocks.

Turned out it was -partially- blown fuse.
Check you fuses or replace, at least you can eliminate that as a possibility.

KP

Kona, if you have switched the speakers and it is still on the same channel and have changed amps and it is still on the same channel, then the problem is coming from before the amp. From your description, it is a subsonic frequency. When you used a different pair of speakers, they may not respond to a frequency that low and made you think that they weren't having the problem. Disconnect the preamp from the amp's inputs and see if that stops it. I think it's in the preamp from what you've described. Since it happened when the volume was all the way down, that pretty well eliminated any source problems. When you changed amps, that eliminated the amp. When you switched speakers and the problem remained on the same channel, that eliminated the speaker. The preamp is the only thing left. Try a different preamp.