So far I have tryed the Telefunken NOS, Dutch Amperex NOS, RCA, GE, Tesla, Sovtek (WA, WB and LPS versions), Ei and Electo-Harmonix. To my ears the Electro-Harmonix are very good but the inexpensive Ei 12AX7s that I bought from Parts Express for $6.50 each sound the best.
