Aric Audio Systems Unlimited Preamp

@rocray - I definitely second what Kalali posted regarding the equipment on either side of the stream having a large bearing on what tubes may sound best in the Unlimited. I want to also suggest the Gold Lion 12AU7, and 12AX7 (I haven't tried the GL 12AT7, and am not sure if they make one.....they must though?). The Gold Lions I've tried so far have had very good definition to them- without leaning towards too much emphasis on one frequency or another. I found they have a very balanced presentation, however each does provide just a little "coloration" which I find to sound very complimentary so long as the rest of the system is fairly balanced/neutral. I hope this helps! Aric 

Rocray,

I have some Sylvania JAN 5751's that sound very nice in the unlimited.  The 5751 bring out the mid-range and the bass a little bit more and are supposed to have less gain.  They're very nice.

 I also have some Mullard 12a x7s. For all they're hyped up to be I didn't exactly find them all that great.  Maybe I just need more time with them.

@aricaudio,thanks for the recommendation on the signal tubes. I just popped in a pair of new Mullard 12ax7's. I find them to tighten up the bass a bit,and bring the mids just a smidge forward. So far they seem to work nicely. Always trying to kick it up a notch!

@jgoldrick,thanks for your recommendation as well. Funny how different things work for different ears. I will definitely look into a pair of the Sylvania's. I swear,this hobby is a sickness. ;)

Look here and scroll all the way down.
I found this chart to be pretty darned accurate . Maybe it will help some of you here .
This is for NOS tubes not new stock. 
https://tubeworld.com/12ax7.htm

Here are a couple of comparisons/reviews for the new stock 12AX7 tubes:

https://www.thetubestore.com/12ax7-tube-reviews
https://www.tubesandmore.com/tech-corner/12ax7-comparison-current-made-tubes