Bought mine used so it's got whatever tubes came stock with it probably. Haven't cracked it open yet so I'm not sure. I've got some Ruby's, Sovtek's and some GT's lying around that I figured I'd experiment with. Haven't gotten around to it yet but I'm kinda getting the itch to try it. Pretty hard to go wrong with Mullard's and Telefunken's. Thing is, there's more to tube pre's than the tubes. Lots of other stuff in front of and behind the tubes that all have an impact on the sound and performance. IIRC, Jim's mod swaps a number of the caps to film versions to make the pre "faster" - basically it responds quicker to incoming signals I think. So just changing the tubes may not have a big impact on how the thing sounds. Just guessing here, though, since I've not tried it yet.