Tube rolling my preamp with 6922EH

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

"Warmer" and "darker" are colorations and exactly the opposite of "transparent". If you want "warmer" sound and less highs, a pair of NOS Mullard, Ediswan or Brimar tubes(British manufacture/coloration), ie: (http://www.upscaleaudio.com/product.asp?itemid=282&catid=42) will provide that without a sacrifice in imaging or soundstage.

I should have mentioned: With regards to the "focused bass" you're seeking- That's usually more a function of the amp/room/speaker/cable/interconnect combo. Of course the preamp plays a role as well, but the pre and tubes can provide as accurate a signal as possible, yet the bass end up sloppy via a lack of synergy elsewhere.

Any tube you put in the sgnal path invariably changes thes sound. However, there are more and less sensitive circuit respones to rolling.
If I am getting you- You really want completely neutral 6922s. Then I suggest the least expensive real NOS fom 1984 maybe but NOS Jan Phillips you can get at reasonable prices. Even the old cheap roll JAN is now expnsive from some dealers and there are a few that naturally cost more, for example the 7308 JANs .
The usual demands for improvement are complicated due to the excessive demand. The prices are almost impossible to figureband importantly don't tell you the most important issue. The tube you buy must be in excellent, non microphonic, quiet condition.
No matter what else the tube is supposed to be good at, it's useless if it's noisy.
The prices are obscenly high. Thus excersize caution- you have been warned. - Buy them from a very repuitable dealer who can tell you a bit more about the sonics but no one will declare what the tube will do in your rig.
IMHO The German tubes typically give you a fast clean attack and accuracy. They can be very sterile sounding so be sure to know what you know what sound you want before plunking down a ton of money.
The British are as mentioned on the warm side and a bit mid range emphasized. There is my favorite the Mullard Mil. spec CV2943 or E88CC-01 which is put in the 7308 category It is the only british tube I have heard that sounds more German than British. They are scarce but I think uncle Kevin had some. They cost too much at this point, for me to pound a shoe about them.
I would not be afraid to save some money and buy 6DJ8s they are much less as a rule and there is a wider variety of good tubes available.
Good Luck and buy them from a stand up dealer only. Because these obscenely expensive tubes will be sold without any hesitation the worst condiion really the old reject quality. Seller that are just part time, have no testing instruents and is a grandmother named Frank. You know at that small auction operation.
Disclaimer--- I have become a regular with a couple of dealers from that site so no not everyone is out to steal.

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?