What would be the best way to take some of the high end off of a booster like the SHO or AMZ Mos Booster?

Cap from signal path to ground, between the input, and output .
 Put them on the circuit side of a DPDT or 3PDT stomp switch, or it'll attenuate highs of whatever goes through that effect box, effect on or off.
 Try different values in different places, just before the output cap is a good place for a high end  rolloff cap to ground.

LArger cap vlaues from signal to ground allow lower frequencies to be shunted to ground...bigger rolloff cap/less highs.
 I'd start around .0022uf and work up.