I’m thinking about doing a very similar thing, although I never actually owned a Maths in the first place.
Check out the Happy Nerding 3xMIA, that’s a cool 6hp attenuverting mixer that you can do a lot with!
For envelopes, you could also look at the Intellijel Quadra (you can build a 2hp version of the expander too, if you’re that way inclined) and maybe the NLC Triad, but that doesn’t have voltage control.
The quadra, plus 2hp expander, plus 3xMIA would get you 4 AD/AR envelopes plus 3 (6 kind of) attenuverters in 20hp, not bad! I think I’m going to combine the Triad and MIA with a Befaco Rampage. That’s 3 AD/AR envelopes and 3 attenuverters in 14hp.
[DISCLAIMER- this is what i’m planning, I’ve played around with most of these, but don’t actually own them and can’t fully vouch for them]