Hey PrimalScream91,
I'm not really a big fan of the Sovtek brand personally. I haven't actually tried those in any of my ADA preamps, but I did try a few in one of my vintage Fender twins, and they only lasted about 3 minutes. That's because it only took me that long to find out that they didn't give my amplifier the classic clean tone it's known for. They break up WAYYYY early and distort too easily, so I don't imagine they would be suitable for a preamp like the ADA's.
The Ruby's as MJMP mentioned, are Chinese tubes, but one of the few Chinese tubes I would look into. I would more than likely look at the Higher priced Ruby Tubes, and those with ratings suitable for HiFi equipment, The 12AX7ACZ or the 12AX7AC5 you mentioned, might be along those lines. I have a trio of the 12AX7ACZ's in my 3TM, and they aren't bad at all.
Yes, I probably got a bum set of Mullard RI's that went South after 3 months in my Classic, but I got those through Musician's Friend, so who knows how long they had them sitting around? The Mullards in my MP-2's I got from Doug's, and they are still going strong. I'm still tube-rolling with the Classic to see if I can get some more pleasing results with it.
To my ears, short plate tubes seem to have a lot of high-end harshness, so I look for more low mids and bass, and that's where the long plate tubes seem to generally fill the bill. Still, there are a few short plates out there that have some enhanced low frequencies, but not many.
It's a lot tougher these days to shop for tubes since you can't really go into a music store and roll a few tubes through your amps like we used to do in the old days. That's one of the priceless benefits of this Forum. Players here are always trying different things, and you can get a pretty good picture in your mind of what you are looking at because chances are, someone here is already using it.
Harley