Thanks for the reply, Larry.
I put the kit away for a while, but decided to take it out again after buying a VHT Special 12/20RT that could use a good mid-boost.
Comparing this to a Tubescreamer again, I noticed another major difference. The high-pass filter in the TS's clipping-stage is an RC pair connected to GROUND on the schematic.
In the Mid Boost schematic (which did not come with the kit, BTW), both RC pairs are connected to the 4.5V source voltage tap. R1 + C2 are very similar to TS specs, and my guess is they form a 2.2 kHz low-pass filter -- perfect for a bright upper midrange boost.
I recalculated the other pairing of R2 + C3 to be 2.1 Hz high-pass filter. Because of the polarity of the cap, looks to me like it's just keeping DC and ripple respectively out of the signal path.
What I don't understand is what happens when the RC networks are connected to 4.5V instead of ground? An active boost?