What 6922 tube will improve bass definition?

If you are interested in NOS, check out Tubemonger. I've purchased from them and been impressed with their service and quick shipping. A range of tubes and prices.

Tubemonger is great but if you want a new production 6922 I would try the Gold Lion "reissue". Jim McShane over at tube asylum is a very good dealer who stocks them.
The alternative if you can find them, are indeed NOS, but man are they getting scarce. What you want is a full but clean sounding tube. These qualities are tough to find in a single tube. I can tell you my favorite is the Mullard CV-2943 a unique sub. It is not rich and warm like most Mullards but rather a strong clean signal. Other good one are the perrenial favorites like Amperex white lable early 60s, Siemens West German but they run into big bucks. If you want warmth then most Mullard tubes will do it. However it really sounds like you want the GL "reissue"

The early to mid 1960's Amperex USA White label 6922 is the one to get for the money. Great, defined, strong bass, neutral mids and beautiful highs. The good, older Mullards from the late 50's through the 60's are smooth and sweet, but more lush than the Amperex I mentioned. The bass with the Mullards is full and rich, but not as fast. The Amperex 7308 US white labels (1960's) are also nice, but a tad drier sounding than the 6922 versions and more expensive. Telefunken 6922's are fantastic in the areas of imaging, detail and clarity, but do not have as much "body" to the mids and bass, and are BIG money.