Jim...I found that my T5000 sounds best with a good quality GE 6201 (12AT7WA) tube...they are plentiful on eBay and not very expensive...if there is a multi-tube auction for a good price, then that might be your best option, as different tubes of the same design can perform differently, so having 2-3 tubes is a plus...
*some examples:
2 Tested GE 5 Star 6201 Tubes 12AT7 on eBay.ca (item 360339241019 end time 24-Feb-11 17:19:19 EST)
2 Strong GE Gray Plate O Getter 12AT7 / 6201 Tubes on eBay.ca (item 120657852788 end time 06-Feb-11 08:48:37 EST)