I can vouch that the B3K combines well with a Rusty Box, as long as you dig a really clear and punchy drive rather than thick vintage-style compressed drive- they both add alot of high end and upper mids (though this can be EQ'ed out to some degree with the Rusty, it is also kind of the "special sauce" of the pedal in my opinion!) and both are fairly tight and punchy (though the B3K can get pretty thick in the Fat setting, it doesn't muddy up like a Tubescreamer or many fuzz pedals in the thick settings). I personally wouldn't dream of getting rid of my Rusty as its tonal profile is like magic to my ears, but if you are mainly using it for an overdrive sound and its not really doing it for you the B7K might be where its at for you. I prefer the subtler aggressive tones of the Rusty to the B3K and the heavier driven tones of the B3K over the Rusty, but it is all a matter of taste. The B7Ks EQ is also certainly more versatile from what I've heard of it.
I doubt you'd be disappointed with the B7K given that you are already considering trading out the RB- it won't give you that Rusty tone, but will likely be more transparent and versatile while keeping you in a similar sort of tonal space...
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