I suggest, even urge, you to invest in a pair of Black
Treasure CV 181. I've used them in amps, i my current
preamp and IMO they beat my stable of some of the "best
of the best" old stock 6SN7.
I buy mine straight from China for less than $130 per
matched pair. No need to pay more.
http://www.aliexpress.com/item/Shuguang-Treasure-CV181-Z-
Vacuum-Tube-Matched-Pair-Replace-CV181-CV181-T-
6SN7/32403414804.html