I have a pair of NOS Mullard 'bugle boy' and a pair of NOS RCA's.
The Mullards are sweeter sounding,the RCA's cleaner sounding.
Im using the Mullards at the present.
If any owners of this preamp are still using the supplied Chinese 12AT7's,get some good NOS's in there and you will be in for a shock.
HUGE improvement.I've been messing w/tubes for a long time but never had this type of positive sound change from tubes.
You can get some good 12AT7's off eBay for a low price.
The 12AT7 isnt in much demand, so they go at a lower price.
Just make sure the seller has a few + tube selling feedbacks.
Also,I would get a hold of Tyler who I mentioned above.He's a great tube dealer and has a huge stash.