What tube preamp to go with McIntosh MC275?

Hi,
I currently connect my DAC directly into my power amps.  Both my power amps has volume controls.  They are McIntosh MC275 and a customer built 300B.  Actully, I think they sound good, but I am wondering if it can be improved further by adding a preamp.  My speaker is Harbeth 40.1.  I don't need louder volume, so, I am purely looking for something to improve the sound.  If you ask me what I am looking for, I think bigger soundstage, more life-like, and more slam in the bass, I guess.

Any suggestions?  I can see many preamps use 6SN7 tubes, but my custom built 300B already have 4 6SN7s, I am not sure if it will make much difference by adding more 6SN7.  Would 45 make a good preamp tube?