Recommended tubes for LTA Preamp

Interesting. 

The LTA can use either (6SN7/12SN7) with the flip of a switch.

DeKay

The VT231 is the military version of a 6SN7GT. 

I replaced the 12AT7s with Telefunken ECC801s. I replaced the 12SN7s with CBS/Hytron VT-231 per a recommendation from Brent Jessee. I definitely recommend reaching out to Brent. He is incredibly knowledgeable and extremely helpful.

In my system, the tube changes were a dramatic improvement in sound. There was better top and bottom extension. The sound got even more dynamic. It wasn’t cheap, but the rolling to me was well worth it.

When I had an LTA integrated with a preamp section, I contacted Brent Jessee at tubes.com for his recommendation.  I told him sort of what sound I was looking for and he gave me great advice. 

Siemens 12AU7 NOS are a nice improvement in clarity over the stock tubes.  Lowers the gain a bit as well, which is helpful unless you are running LTA amps.

My favorite are NOS Telefunken ECC801S diamond bottom made in Germany. I also like NOS Siemens ECC801S.  The Telefunken are pretty pricey. 

I like the Brimar Yellow T 6060 12AT7. They have them in stock at The Tube Store. The Brimar are warmer and slightly less detailed than the Telefunken and Siemens.

If you are looking for a brighter more detailed sound, I'd get one of the first two. If your system is already bright, I'd try the Brimar. 

That's been my experience, anyway.