VTL Pre tube roll…or suggestions on 12AU7s

Wondering if anyone has swapped their tubes in their VTL preamp.  I have the 6.5 II.  It doesn’t sound bad with the current tubes, but, as audiophiles, we audiophile.  Any suggestions on 12AU7’s, otherwise?

Also curious if anyone has input on VTL’s claim in the manual regarding damage occurring from using non-VTL approved tubes…seems a bit odd to me, as I’ve not read that in other tube pre’s I’ve had.  Or that I recall of, my guess is it’s a general legal disclaimer.