LTA MicroZOTL tube upgrade

What do you have for stock tubes?  I haven’t tried Ray Reserve, but they get good reviews. 

I’ve tried Brimar Yellow T 6060 (12AT7) and they are warmer sounding. Not what I wanted. 

My favorite tube is Telefunken ECC801S.  Second favorite are Siemens ECC801S.  Neither of these are warm.  

My stock tubes are Mullard 12AT7 M8161 and Radiotron Electron RCA.

Since I'm new to tubes, I assume these first ones have the biggest impact on the sound signature?

Both sets of tubes make a difference.  I first experimented with the input tubes.  I bought my preamp lightly used and the prior owner had already upgraded to the Telefunken ECC801S.  I ended up preferring them.  They're pretty pricey.  The Siemens are generally less expensive if you can find them.  Both are new old stock.  Both offer excellent detail and are in no way warm sounding. 

The 12SN7 or 6SN7 also make a difference. I think I'm currently using Sylvania Gold Brand 6SN7WGT tubes.  New old stock from eBay.  I've tried several different types of NOS tubes. 

I've got the latest microZOTL; I liked the stock 12SN7's. What I did need to change though were the stock 12AT7s - they were really holding the sound back to the point where I was ready to send back the preamp during the trial period. I told Jacob at LTA and he sent me a few alternatives to those, and 1 of them, tall Dutch Mullards, did the job perfectly and I'm very happy with the preamp now. 

@larsman I have Mullards 12AT7 so probably the same they sent you.