I replied to your other thread as well. I recently got a B3K, and find that they pair pretty well. The Rusty tends to bring out alot of high end noise and other harshness from things before it in a chain in my experience, so the B3K seems to like being placed after it at this point (I haven't tried the B3K into the Rusty when using it as a preamp straight into a power amp yet though). Both the Rusty and B3K add alot of higher harmonic content, so they are more parallel than complimentary in that sense, but I find using the B3K allows me to cut Treble and Mids a bit on the Rusty in a way that I like to get two useful and complimentary tones. The tone I get out of the combo of the two is verging on a thicker and punchier version of a Rat-pedal like distortion though, so might be to agressive for your needs?
I haven't tried pairing an Agro and the Rusty, but I could see potential there as well from what I've heard of it- the Rusty is pretty good about adding its character to anything it is paired with in my opinion. I think the Tone Hammer pre might be a better complimentary tone to the Rusty than the Agro, though it would be kinda silly using two high voltage pres like that just for dirt effects (or not!).
I (and most other Rusty Box users) am with you on the relative uselessness of the Boost channel- with the pedal set clean as I tend to like it, the Boost doesn't really get driven enough and adds too much volume). The only use mine gets is when pushing into my envelope filter in place of (or combined with) my fuzz pedal...
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