They're not just for EQ. They're icky without diodes, but you can get all of the good parts of tinkering with diode-free TS experiments by just putting big LEDs where the old diodes used to be (don't forget to put them in the right way!), and the OPA2134 is definitely not a diode (!). The clipping level is very amp-dependent - some want more or less clipping going in, as would be expected depending on how they take cleaner or messier signals - but I'd definitely try putting in a switch for different diode pairs and putting an op-amp socket in to try more op-amps in easily. As it takes a bit of volume knob drop-off to make much difference that the gain knob wouldn't make on its own (due to the way that modern amps are built - as opposed to decades ago when your only real hope for heavy distortion was a hot pickup), expect to see much more difference... especially since any modern TS-type pedal will have at least an output volume knob that does little but change how much sound is coming out of its tail end.
By all means, swap one out for an EQ that looks like a frowney-face centered around 700-800Hz... then follow it with a compressor... then put a simple clipper before and after various bits. There's a teensy bit more going on.
Jef
