The "basic" story on the low gain vs high gain.
I got in early on the VTA board, used it and liked but the gain really is just to high.
(makes the amp noisier then it needs to be and to sensitive for most people there)
I set out to change it, an make it more tube roll friendly, the story went from there.
(want/need any more info then that you can PM me)
Basically the Low Gain version is "normal gain" and the High Gain is, Super High Gain.
The Low gain version is just fine for passive pre/attenuator as well, it's not low gain.
I would recommend the "low gain" version for any VTA build, it is smoother, easier to
use an it opens up quite a bit of different tubes that can be subbed/rolled on into it.
Also on the "low gain" version watch out for R13 an R14, anything higher than 5k
there and you'll get to low of voltage to the plates of the middle tubes, I say this
because I've seen some boards, amps built with to large of resistors/voltage drop.
(that's the RC stage resistors feeding the input tube there voltage on the boards)
With 30k R13/R14 combined with the power supply RC stage you
get less than 120v on the 12BH7 an less than 140v on an 12AU7.
(you can run either in the center spot, pretty low on the 12BH7)
Unless your running an direct coupled stage those voltages don't
need to be that low, the tubes will perform an sound better with
more voltage to there plates, and that has been what I've found.
So yes the 30k R13/R14 is lower then I'd run it, at 4.7k there it'll
get about 160v on the 12BH7 and then about 180v on the 12AU7.
Those are "better" voltages and is what I'd run if using that setup.
Not saying "do this" in the following section here, but.....
Now me personally I run an "Little Different" plate load and cathode
resistors on that center tube since this low gain mod was first made.
It's tweaked to better run a E80CC but still run the 12AU7 an 12BH7
in there as well and perform as they should just like the other tweak.
(all those tubes are rollable in that center spot, I prefer the E80CC)
So on mine it is the same 4.7k at R13, then 43k at R11 an R9 at 1.2k
(so that's, RC stage resistor to that section, plate load an cathode)