Tube whistling

Hi, I have a Counterpoint SA7 preamp (the early model) not the 7.1.
It uses (2) 12au7 tubes in the line stage and (2) 12ax7 in the phono.
If you are familiar with this unit, then you know about its problem with microphony.
That is something that can be reasonably tamed but another problem the unit has is a tendency for the line stage (12au7) tubes to ‘whistle’. That is, a high pitched sound introduced into the signal that changes pitch and tends not to go away until you actually tap the tube that is causing it. It comes and goes, is at a very high pitch and not loud but definitely can ruin the listening experience. I was told that there is a modification that can be performed to allow the unit to run 6DJ8 tubes in the line stage that will alleviate this problem. My first question is does anyone know how to tame this problem without such a modification? Let me point out also that I have tried tube dampeners, isolating the unit with special feet and how loud or quiet I play the stereo has no affect on the outcome. My second question is has anyone performed the modification that allows the use of 6DJ8 tubes within this unit? I do not wish to purchase new tubes just to see if that will cure the problem for a short while unless of course that is actually something that someone of knowledge is going to tell me…
Thanks
rick