Aloha Slowhand,
I am using Svetlana 6L6GC's instead of the 5881's. Several guys on the Tube Asylum have said that the Svet's are really good. I have to agree so far. I had ordered RCA Black Plates, but the consensus on the Tube Asylum is that they are over-rated especially for the price, so I cancelled them.
I am also using a Mullard 5AR4 rectifier.
So far I have rolled the Syl 6SN7GT's and the Ken Rad VT231's (also in military boxes). Actually, I just sent an email to see if he has anymore.
The Ken Rads are awesome, but the difference I heard between the Syls and Ken Rads could also be attributed to break-in of the preamp. I put the Syls in from day one and just switched to VT231s yesterday. I also tried a couple of tweaks to my system last night that really elevated the sound, so if I were to stick the Syls back in they may sound just as good now. Will try them again in a month or so.
But I gotta tell ya, this thing is really singing. I never listened to it with stock tubes, but with the tubes I just described, this pre is simply amazing.
Aloha,
Warren