That could happen if you are using a LIN pot instead of a LOG pot, or if you have the LOG pot wired backwards.
I did use a 500k LIN pot indeed, because I did not have a LOG type, so it's a long travel from 50k to 500k and with a LIN pot nothing much interesting happens in between.
The most interesting sounds are really in the 47k, 100k and 500k positions, and I will eventually go for the 3 position switch to keep it a true "Tri-Vibe".
I guess the size of this circuit is as much as you can pack in such space without going to surface mount devices.
Yes, for me it probably is.
I made a PCB with the same size for Mark Hammer's "The Woody" and with 2 Quad opamps and quite a lot of caps and resistors that also was a tight fit,
but I see it as a challenge to make rather big circuits fit in a 1590B enclosure.