I have got a great sounding '71 Park 75 but want to tweak the tone a little. Sometimes the midrange is a little strong under a MIC. The culprit could be my G12M25 loaded Bogner 2X12 (@ 8 ohms) having a Plywood back, or maybe that the amp has been tubed with sylvania big Bottle 6CA7 power tubes. I am hoping some preamp tube juggling might help me out. Which of the following preamp tubes and in what placement would Y'all think might deaccentuate the mids a little or optimize the tone? Telefunken 12ax7/ecc83 (I have 3) RCA 5751 RCA 12ax7 JJ 12ax7 Other Recommendations?