Yep, I've used the GT12AX7-C in the V1 and V2 sockets. The 12AX7-C absolutely KILLS the JJ ECC83s. It yields a much more dynamic sound.
I recently swapped my GTs out with two Tung Sol 12AX7s. I haven't decided which I like better. The Groove Tubes really bring my clean channel to life, but...