What 6922 tube will improve bass definition?

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"

No one has characterized the GL as a hard sounding tube. It is invariably described as being on the warmer side of neutral, this is why I suggest that tube. I know very well what Sovteks sound like and they almost alone sent me into tube collecting hobby for the last 10 years. I think most of the Chinese stuff suffers many of the same problems. Now and again you want something a bit on the cleaner and stern side for a warm sounding piece of equipment. I had an Audio Aero Prima CD player once whch needed a 6922 type tube. It was kind of fluffy and muffled at times. I ejected the tube that was some piece of junk and found my favorite as above which I mis typed it is a CV-2493. Kevin Deal had them for a while as did tube monger but I am not sure if they still do. They are not warm so I wouldn't suggest that for you, but if you are really curious and have the money there is a dealer in Sweden who always seems to come up with them. He is on Ebay I forget his exact moniker but if you search you can find him.
BTW the other tone control at your disposal are your cables. My system was very bright and hard sounding at one point so I swapped out some key interconnects and speaker cables to come up with the sound I wanted. Again there is a price for that. I used Cardas Golden Refs (used) with super results. That is another bag of worms I will leave at that.
Good luck