Just my .02, but if you're looking to buy a zero turn anyway, I wouldn't look at it as filling in the gaps for the BX, but instead having it wholly replace how you cut your lawn. You probably feel some attachment to cutting with the BX since it was one of the purposes you bought it for, but in my experience a zero turn is better in every way for cutting grass. Even on steep hills where a BX in 4WD was almost necessary, the steering wheel ZT's do an excellent job.
I would sell the MMM deck and buy the right ZT for all your grass cutting. Leave the BX for loader and 3-pt work. You'll have much better ground clearance with that deck off too.
Switching to mowing with a zero turn on my property cut my seat time to 1/3rd.
Edit:
Forget to answer your actual question
If you don't have any major hills, an orange friend for your BX would be the way to go. If you have steep hills, I really like my cub cadet pro 960s. Cuts on, and will turn up my 26 degree slopes no problem. I think a lot of people just can't take the ZT's with a steering wheel seriously, because they have a steering wheel. It's a shame because I demo'd 4 different ZT's on the slope I cut (Ferris, Kioti, Exmark, and Cub Cadet). The Ferris, Kioti and Exmark were all lapbar machines and were superior on the rest of my yard but would not hold my front slope. The exmark came close if you went real slow, but inevitably, as soon as it slipped, the front casters would turn down the hill and there you went. The cub cadet with the steering wheel just holds and holds.
