I build a Supa Tonebender according to this schematic: http://eu11.stripper.jp/pulcino/blog/archives/000024.html
. very similar circuit to the Big Muff, and you can probably use a Muff/Beaver PCB to build it (or make your own like I did). As with a Muff: long sustain, lots of gain. But still, with some careful tweaking you can get nice gritty lower gain distortion sounds with some actual dynamics in there. The Steve Hackett signature Tonebender (see http://filters.muziq.be/model/colorsound/supatonebender
) is probably very similar, although I have not seen the insides of that box.
However ... Hackett used more fuzz boxes than this one, and if you want to approximate the early Hackett sound (as in the first albums of Genesis), you may be better of with a Germanium fuzz like the Tonebender MKII ...
Quick edit: my Supa Tonebender has plenty output for me (considerably more than unity gain).