Tube rolling my preamp with 6922EH

My preamp is the Bob Carver Sunfire vacuum tube control center which is running 3 stock sovtek 6922's. I know better tubes will make an improvement and am trying to find some good tube alternatives.

Like to achieve a warmer, darker and transparent sound with a focused bass and less highs.

Thanks for all the suggestions. Mechans; you've hit on what I'd like to achieve, which is the most realistic sound with tubes for the biggest bang for the buck.
Would love some of those recommened rare NOS, but want to keep things as affordable as possible. Therefore, for consideration my next question becomes, if you wanted to go a step up from the 6922EH to most affordable NOS today, what choices are there that some of you may have discovered?

I'm out of the ball park for expensive rare NOS. Would like to keep under a few hundred dollars for all (3) tubes.
Newbee, please tell me more about the Old Yugo's and why you like them so much. What time periods/years were the ones you like manufactured? How would you describe their sound qualities? What weaknesses, if any?