Remember, the Max 12 is a dual mono amp that cannot be bridged (i.e., needs two 4ohm cabs to reach its full power potential). The hi pass filter is set at a lower level on the Max amps and 9, so they sound fuller and fatter down low... very nice. The 9 sounds WAY better than the 6 all the way around... a bit louder, but more importantly, more full and fat down low.
For that cab, it would be the Shuttle 9 all the way for me. The bright, tight, modern voicing of the 9, along with pretty good power specs, should match up well with the dark and relatively low SPL Ampeg cab you have. IMO!