In an attempt to fix a volume loss issue in my HRD, I recently replaced all 5 tubes with a brand new set of JJ's. I believe that problem is fixed, however now the clean channel breaks up considerably around 2 on the volume pot (should be around 4-5).

As recommended by eurotubes.com, I biased the tubes to 75mv (in the recommended 70-90mv range).

I have no idea what's causing the problem.

Could it be socket issues? unclean connections?
JJs by nature have higher gain. At least most of them that are out there.

Eurotubes only sells JJs.

Bob's advice of 75% dissipation is a bit high imo. Are you sure you are not talking about that?

As far as the biasing I cannot tell you if 75 is too high or low. I know nothing about biasing Fender amps. The plate voltage is measured in mV but the cathode current is measured in milliAmps. I would think your plate voltage would be much higher (300mV+) and the milliAmps to be potentially lower (20-50 mA). You should bias your amp to the tone not to numbers.

Maybe you can tell us more about how you biased and how you did it. Do you have a bias probe?

Edit: oh and and I'm confused as to why this is a problem
i used the bias test point in the amp. If the power tubes are a matched pair (which the jj's are) then measuring just the test point voltage is sufficient to bias the amp by tweaking the bias pot. The general theory is that it should be set to 68mv. Bob at eurotubes recommends 70-90mv since he claims jj's dissipate heat better.

I use the hrd for jazz tones, so headroom is important to me.

The jj's are a set of 6l6's, not 6v6's, so headroom shouldn't have changed that drastically.