Brothers In Arms
Although I do not plan on this union for my own needs, I believe that others will and could benefit from discussion here.
Depending on the specific application for these devices, the order of them is most worthy of consideration to yield maximum benefit. In keeping with the theme of this thread, the application here will be to use the MS-60B in conjunction with the B3 used as a preamp.
I believe the main reason one would want to use both devices together would be the existing limitation of only 3 chained effect models on the B3 versus 4 on the MS-60B, and the MS-60B's 1-switch design does not offer as much "Live" flexibility as the B3's 3-switch design.
If this were my rig, I would want the B3 to host 3 popular effect stomps and I would relegate the more consistent preamp duties (i.e. amplifier models) to the MS-60B. In my world, stomps are most enjoyed in front of a real amp and not through its effect loop if one were available. With this said, my wiring chain would be from instrument, to B3, to MS-60B, to power amp, all via 1/4" cable. The consequence for this is that now I will need to use an external DI box inserted between the MS-60B and power amp should I need a balanced XLR out, since the B3's XLR will not have the complete sonic picture.
(Should this not be acceptable/possible to you then you certainly can, in less-effective arrangement IMHO, order the brothers the other way and use the B3's XLR out.)
Under my device arrangement, I would configure the down-stream MS-60B to be a different amp/cabinet upon each pedal click, as the MS-60B can be programed to cycle through up to 4 different patches, and the up-stream B3 would be treated like 3 independent stomps before the amp that can optionally be swapped-out via B3 patch change.
Regardless of the chosen device ordering, be sure that the output levels from the up-stream device are in the ballpark of "instrument-level" as not to overload the input of the down-stream device, else unwanted distortion may occur.
Cheers!