tube amp/flutter in speakers?

Are you sure its not the components upstream from the amp? You can check this effect by simply removing the input connections from the amp, and unplugging all of the gear that is upstream.

I have seen ground loops / ground problems cause this...

If it is the amp -- it seems like a power supply issue -- this may happen due to a bad diode, regulator tube, or capacitor. There are certainly other things it could be - these are the most common ones I have run across...

Good luck -- and follow Sean's advice if you can't quickly figure it out...

Unless it's a rare output transformerless (OTL) design, your amp has an output transformer between the power tubes and the speaker. This transformer will not pass DC unless it is seriously defective, in which case you'd likely have more severe symptoms such as smoke and overheated output tubes.

If it's not coming from an upstream component, then as mentioned above, low frequency oscillation is the likely culprit.

I agree that it's time for a trip to the shop.

Ghostrider, don't believe the old wives tale about transformers not passing DC. It's a lie. They will pass DC, just not anywhere near as effectively as they do AC. Sean
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Sean, was that a KISS concert or AC/DC where you so the flash out of the speaker?

if this helps, i have tried other speakers on the amp and no problem. could the heil driver be the culprit.