I just wanted to post a quick update. I've rolled the following tubes through my preamp:
1. 12AX7EH
2. GrooveTube 12AX7M "Mullard Reissue"
3. Ei Elite 12AX7
4. Sovtek 5751
5. A trio of Telefunkens a friend loaned to me
I have determined that the TS6550 "reissue" power tubes are not to blame for the brightness. If anything, they are a very neutral tube--they show changes in anything upstream VERY well. They can soundstage well if fed properly, and seem to take on the characteristics of the preamp tubes feeding them more so than my 6550EH. To me, this means they are a fairly transparent tube.
This is a good quality, and for folks with tube preamps you can use the pre tubes (much cheaper) as tone controls of sort. However, folks without tubes upstream who want the tubey sound might want to try these tubes before buying them--they are a neutral, modern sounding tube, and some folks may not like that.
I haven't formed an opinion one way or the other yet. The best matchups I've found so far are Ei 12AX7/TS6550 and Telefunken/TS6550. The first pairing is very liquid, smooth, and a bit laid back. Putting the Teles in adds a bit of sparkle and midrange tube magic, at the expense of some soundstage focus and air on top. I've only had the Teles in one day, so this is a bit premature...
I have some other pre tubes to try, too; some 12AX7 Mullards and some 12AX7 Mullard Gold Pins. I am also ordering a few more new-production tubes, and may have a trio of NOS TungSol 12AX7's to try soon as well. I'll keep everyone posted, but I doubt my impressions of the TS6550 will change too much. I'll post final impressions, as well as a verdict, in another thread.