The BlueBuster is based on the Bluesbreaker amp. It is meant to have the master volume cranked. You can pretty much read about this regarding any Bluesbreaker style pedal. That said there are a few things you can do to increase the volume even higher above unity. You can add a gain stage like the Afterblaster onto the foot switch. You can simply change the clipping diodes to LEDs or you can double up on the silicon diodes increasing the forward voltage. Either way cranking the Volume is part of the excellent tone.
As for the Muff circuit the Volume drop could be apparent since it features a fairly severe mid-scoop, while the Blues Buster has a mid-hump. It is hard for me to say without hearing it in person but I would try other pedals.