New spec calling for 5AR4 instead of initial 5Y3 rectifier in CA-906

Linestage is stereo dual 6SN7 (New Tung_Sol). Power stage is 5Y3 to dual 6V6 to 6SL7. I like the sound of the newly recommended 5AR4, except that with my NOS (Mullard, RCA, Ken-Rad, respectively) tubes it emits low (-60-70dB) scratchy sounds (noisy). Subbed the Mullard with a new Sovtek--same noise. Factory recently says use all JJ in power stage (just ordered, they're built sturdy I know, for guitar amps). If I use my CBS Hytron JAN 5Y3GT, no noise at all. (I tried another NOS JAN 5Y3 and it DID develop noise, so I'm guessing it's just bad.) Ugh! I wonder if the increased plate voltage and doubling of current from 5AR4 rectifier is creating noise somewhere else (6V6s? 6SL7?) Unit was totally re-capped last month, so I don't think caps are acting up. I can't find a schematic, so I'm asking for anyone who knows tube amp power sections... The 6SN7s aren't the culprit (I subbed them). The intermittent scratchy sounds that appear are independent of volume selection. Doesn't exist on mute of course, but appears on first notch up in volume, and stays the same at any level. I've asked the factory, no response yet.