Tube rolling LTA MZ3

wondering if anybody has done any tube rolling with a LTA preamp? Just got one, my sickness immediately makes me wonder if I can make it even better! Anybody have ideas on weather the 12sn7 or 12at7 position would have the most influence on sound?  For me, (in my system, my ears) this pre is an upgrade from Pass Labs XP12. So much more dynamic and engaging.
thanks for any perspective, Ted
Yes. I found the stock genelex 12at7s a bit grainy compared to the mullards I have in there now (cv4024). Also RCA 12sn7s from Brent jessee (not expensive) made an improvement too but I’m still evaluating that.
modern day chinese or russian small signal tubes are often quite bright and strident compared to old stock counterparts from usa europe or japan
LTA does include NOS tubes with their full size Microzotl preamp and they are usually GE or RCA. I found these tubes sound great and can be found without taking out a second mortgage.

From my rolling, the circuit was far more sensitive to the 12at7 than the 12sn7 and the stock small tubes exhibit glare. 
I love my Microzotl preamp. Prior to this I has a Decware CSP3 with all the upgrades. Got the upgrade itch and tried the original Schitt Freya after reading all the great reviews. Kept it in my system for several months and rolled quite a few 6SN7's through it. Fortunately I owned a few already as my Melody M845 amps use them as well. I never could get the Freya to sing for me. Wanted to like it but just didnt have the synergy in my system I guess. I finally popped the Decware back in and the magic was back. 

The upgrade itch caught again last year and I bought the microzotl pre. Right out of the gate I new the Decware had met its match.

I've rolled quite a few tubes through the microzotl when a pair of the legendary Melz 1578 hole plates came up for sale from a reputable seller. I have a pair of these in my stash and they were the best 6SN7's I had ever rolled in my amps. Unfortunately one of the tubes started to crackle after a few months and I needed to retire them. Before I purchased another pair I figured I would throw them in the microzotl to see if they were a magical as I remembered. Low an behold no crackle. I popped them back in my amps and the intermittent crackle was back.

The Melz are now running full time in my Microzotl with a pair of Telefunken ECC801s in front. For me this combo is magic. 

The main point here is not to convince you that you need to run out and scour the internet for these rare specimens as much as the Microzotl runs tubes at very modest voltages. If you are like me and have some tubes that have picked up that all to familiar popping  / crackle sound consider giving them a roll. YMMV.
I use NOS tall bottle 6085 tubes in place of the 12AT7’s. These are closer to a 12AU7 in specs but will work and sound great.
Ps. 4 6085s for both the AT and AU tubes. It’s important that all 4 tubes in the z10 use the same current
Have rolled NOS Mullard 6201's in place of the Genalex 12AT7s it shipped with. Smoother, less grainy but keeps the detail. Did not like any of the 12AU7's I tried including some NOS Mullards and Brimars. System seemed a bit lifeless.

Thanks to the suggestions here I got a pair of SIEMENS E80CC / 6085 for my MZ2.  Significant improvement over the GL’s that LTA provides.  Replaced the RCA 12sn7’s that LTA provided with a pair of GE’s and I think I prefer the RCA’s. Just got a pair of Sylvania 12sn7’s this morning.  Haven’t tried any of my VT-231’s yet.  Awaiting delivery of a pair of 1950’s Sylvania 12at7’s.