Thomas Mayer preamps - the 6AH4 vs the more expensive 10Y

Don't know.

But I can say this: Cost doesn't necessarily translate to fidelity, or even accurate audio reproduction.

Look inside the preamp. Short signal paths, good quality/reliable parts, heatsinks, a particular method of acheiving quality amplification (such as LIFES from Luxman etc.)

Not sure if someone else will have a suitable answer for you, but in cases like this, it's always best to contact the company directly via email/phone and have them break down the differences between the 2.

I've never heard anything but praise for his audio components, and the build quality looks amazing.

You are paying for the external power supply-tube power supply

The usual responses apply for that.

By the looks of the product line-if you have to ask...

$80K for the top phonostage-perfect for a Techdas.

Phono preamplifiers by Thomas Mayer - VinylSavor - Thomas Mayer

Thomas mayer also owns a tube manufacturing facility called Elrog.

I have not heard his products. But I DIY tube preamps and amps using very similar circuit and components.

In Thomas Mayer’s circuit, 10Y would be better than 6AH4, with a sound that’s more transparent and dynamic (btw this is the difference between DHT vs IDHT in general).

I expect the sonic difference to be quite noticeable in his offerings.