I have a $17 solution for you: RFT 12AT7
Looking in your other thread at the tubes you bought (Mullard/TungSol etc) they all are very similar with a lush/warm/tubey sound but very little weight or punch (if any) in the low end (also I find the Mullard a wee bright for my taste). You should really give a try at the other end of the spectrum, the "cold" german tubes (RFT was east-german). They provide a completely different sound: cold, forward, dynamic and really punchy. I hate bright sound, my ears are overly sensitive to treble but this is from very far my favorite tube. Now my HD650 have a LOT of weight and punch in the bass department. It feels like they're closed headphones compared to when I use the Mullard or Amperex. I would never have imagined such a transformation was possible before I tried it by accident.
The magic trick is that now that they have so much punch I listen at a fraction of the volume I used to (no need to up the volume to get my bass fix) and the treble is perfect at that volume. The sound is dynamic in the treble also but without hurting my ears at all.
It may not end up being the sound signature you're after but the stark contrast with your current tubes will give you a much wider perspective on what HD650 are capable of. Just beware: your head will explode if you try the RFT with the Denons
