Its amazing how those power supply tubes impact the circuits. I have an Aesthetix IO loaded with NOS tubes. I put cheap Sovtek 12AX7LPS from Jim McShane in the power supply but put NOS Mullard EL34 there also. The difference from the Mullards is not minor. For my REF10, will use original Winged C's when the time comes. Honestly, at the ARC Ref level, doubt one can go wrong with picking any of their preamps.