Try some vintage 1940s 6SN7-GT tubes with your BC-21, like the military VT-231 types. They will take the BC-21 to the next level.
In order from the best IMHO (they are all great), I would rank the VT231 tubes (and one other).....
Tung-Sol VT-231
RCA VT-231
Sylvania 6SN7W (about tied with the RCA above)
Ratheon VT-231
Ken-Rad VT-231
Sylvania VT-231
National Union VT-231
Many of these types are for sale on Audiogon alot.
Add a Blue Circle BC61 power cord as well. And put some #2 Vibrapods under the three wooden feet.
I will add that the mid-bass issue could also be in part the CDM-7 speakers.
Also the AQ Slate are very nice speaker cables for the price, but tend to lose a little in the deep bass department, so that may make the mid-bass stick out also. The Slate also have a small hint of dryness. The slight dryness may take a little away from the tube richness. (Disclaimer...I am being picky.) If you like Gilbert Yeung's stuff, you may want to try the BC-92 speakers cables next.
Disclosure....I have no commerical connection to Blue Circle; just a fan.
In order from the best IMHO (they are all great), I would rank the VT231 tubes (and one other).....
Tung-Sol VT-231
RCA VT-231
Sylvania 6SN7W (about tied with the RCA above)
Ratheon VT-231
Ken-Rad VT-231
Sylvania VT-231
National Union VT-231
Many of these types are for sale on Audiogon alot.
Add a Blue Circle BC61 power cord as well. And put some #2 Vibrapods under the three wooden feet.
I will add that the mid-bass issue could also be in part the CDM-7 speakers.
Also the AQ Slate are very nice speaker cables for the price, but tend to lose a little in the deep bass department, so that may make the mid-bass stick out also. The Slate also have a small hint of dryness. The slight dryness may take a little away from the tube richness. (Disclaimer...I am being picky.) If you like Gilbert Yeung's stuff, you may want to try the BC-92 speakers cables next.
Disclosure....I have no commerical connection to Blue Circle; just a fan.