LTA MicroZOTL tube upgrade

I have the Level 3 microZOTL. I’ve used the stock GE 12SN7’s and Mullard 12AT7’s, and the Ray’s Select tubes sourced from Jacob. What I’ve found as the best combination for me so far: Amperex Holland 1960’s medical grade red tip 12AU7, and Ken Rad VT231 6SN7, both available at the time from Brent Jesse. 

In my situation, I needed a little less gain from the preamp. The options were to use the -15 dB gain switch in the menu, or switch from 12AT7 to 12AU7 tubes. I found the Amperex Holland 12AU7’s to sound better than the Ray’s Select plus the gain switch. 

The Ken Rad VT231’s have a very holographic presentation, and good tone throughout the audio band, from solid bass to airy, directional highs. 

@gabriel123 the M8161 seems odd to me, although I am not a tube or electronics expert. That tube is not an AT7 and not even the same type of tube in general (it's a pentode). Is it possible the label is faded or wrong?

Any experience out there with the UL integrated AU or AX7’s? Thanks 

@gabriel123,

I bought a slightly used LTA Microzotl MZ3 from LTA and my final upgrade was to the matched NOS 1940’s (1) Sylvania 6SN7W metal base and (1) Sylvania 6SN7GT metal base, which have different nomenclatures but are the exact same tube and (2) matched NOS Telefunken ECC801S’s. 

And this is after an upgrade from the stock tubes to (2) matched NOS Telefunken ECC801S’s and (2) Matched NOS Sylvania 6SN7GT 1952 ’Bad Boys’ which are also excellent tubes but the the matched NOS 1940’s (1) Sylvania 6SN7W metal base and (1) Sylvania 6SN7GT metal base just brought it to an even higher level of performance.

I also upgraded to the 聖HIJIRI HCI R10 RCA interconnects and the 聖HIJIRI Nagomi Power Cord and at each upgrade the LTA Microzotl MZ3 responded with very positive results each time. And that’s one of the many things I like about this gear is it does respond to changes good and/or bad. 

The only thing I haven’t upgraded is the umbilical power cord as of right now I don’t have the available funds to do so. For the money and what it does, this is the best preamp I’ve ever owned. 

Thank you all for you feedback!

@chickenoregg Any idea what's the difference between Brimar Triple-Mica Black Plate CV4024 and  Brimar Yellow T 6060? What's so special about Marconi B309 and GEC A2900? GEC is above my budget but I found a pair of " B309 12AT7 ECC81 Black Plate Ring getter NOS Marconi UK Valve Tubes" in good price.