I was told by a manufacturer that a good matched phase invertor tube could possibly increase lowend headroom. Now I am learning more and more as I go along my myriad of amps,b ut this is a new one to me. They also said could be a bad power supply cap. I don't think there is anything wrong persay, as the amps sounds great, but I am just wanting the lows to not be so farty. Yes, I've messed with various power tubes. Any thoughts?