I have tried alot of speakers with my Mark IVA head, here is a list of ones I liked and disliked. But for the short version, I would replace the G12T-75's with some Warehouse ET-65's or Celestion G12-65's. The V30's will mix well with those and you have a cab that should work well with both amps.
Here are the cabs/speakers I liked (pretty much in order

1. Mesa Halfback 412 (both sections closed) EVM-12L/MC90 (My marshall 2553 sounds great through this cab)
2. Mesa Halfback 412 (both sections closed) EVM-12L/Eminence MS-12
3. Marshall 1936 212 copy with CL80's
4. Mesa Halfback 212 (top open) MC-90's
5. Marshall 1936 212 copy with Warehouse ET-65's (currently use with my Marshall 2553, but sounds great with the mark IV)
6. 2 closed back 112s, one with CL80, one with V30 (Sounds good through just 1 112, either one)
Here is what I didn't care for (no real order

1. Vox 212 with 2 G12H30's (opened back)
2. Fender Bandmaster 212 with 2 Weber 12A125A's (nice for clean though, open back)
3. Opened back 112 with Weber 12F150 (again nice for clean)
4. Mesa haflback 212 with 2 G12T-75's, top open
5. Marshall 1922 copy with mix of G12T-75 and V30 (horrible cab design)
Hope that helps a bit. I would lean toward closed back and stay away from G12T-75's for the mark IV....