Help with Seduction phono preamp noise

[thrice]

Hey I've got a seduction phono preamp that's developed a problem. I get a low level:

thu, thuk, thuk, thuk

sound out of both channels. it's not bad with the stock tubes, but with a pair of amperex bugle boys it interferes with the music.

It's a fast rhythmic pulsing that is channel independant (shows up in both). I've tried replacing the caps in the PS and the heater PS since someone on the Bottlehead forum suggested it might be "motorboating" but that didn't work.

All voltages check out on the amp except pins 1 and 6 on each tube. Both should measure 70V, but one measures 73 on one tube and the other tube it's 67.

Any help would be appreciated.

Oh yes, I checked to see if B+ and the heater voltages fluctuate either with or without music playing and both a rock solid where they should be (160 and 6)

Any help is greatly appreciated.