Microphonic tubes...

I have noticed that two of the 4 6GK5's in my CJ Premier 10 preamp are somewhat microphonic, producing an occasional high freq whine. I can tell by lightly tapping them lightly with my fingernail. One recently went bad, so I replaced it with a spare I had laying around. System sounds ok - no whine anymore, but I think the tubes are rather old, and I am going to go ahead an replace them all, how do I go about selecting a non microphonic tube with good sound. I can get a set of 5 NOS tubes of either GE or Mullard for cheap - $15.00. Any inputs, other suggestions on which tube might be better from any tube masters out there would be very much appreciated...Tim