OK, so what's so special about these filters?

Well, you can see that the HP uses a follower with a gain of 2.

TYpically, a Butterworth filter based on a unity-gain follower (standard Sallen-Key) would need different values for the pot sections, the one going to ground being twice the one in the FB path.

The usual workaround is to use a follower with a gain of 1.414 (sqrt of 2) or 3dB with pots of equal value.

Apparently, they have gone a little further, which makes the HP filter a Chebychev with ca. 1.5dB ripple, that means that there is a 1.5dB hump just above the cut-off frequency, and the slope at cut-off is slightly higher than Butterworth. Cut the offending bass and boost just above, a nice trick.

The only drawback is that they have to deal with the additional gain, that they need to compensate somewhere (not a big deal anyway), as done by R1/R2 and IC1A.

For the LP, it is even simpler, the FB cap, which is usually twice the value of the one that goes to ground, has been increased by a factor of 2, giving the same C1.5 alignment. Cut the hiss and boost presence.

Nothing magic, but a proof that these guys at Harrisson know what they do, with real hands-on expertise.