[Megatron]

I have used them on 807s in series to drop voltage due to the limited voltage rating of the screen, I used 85A2 IIRC.
couldn't actually hear any significant noise in the speakers.
If you drive the amp well into clipping though it could cause some problems.
ie when the anode voltage drops below reg voltage.

For measuring conducted emissions in the 9kHz-30MHz range we actually use a pair of neons across the line as a noise source.
at 150kHz the emission peak value is pretty much on the conducted emission limit~66dbuV however these are on AC50Hz therefore theres a lot of switching noise when they strike. never tried one on DC, must have a go now its been brought up.