It depend what sort of sound you're after.
The ProAc, and to a slightly lesser extent, the PMC's are a little warmer in their presentation. Both of these will benefit from the control of the Bryston power amp.
The Performance 6's are a little different. Their cabinet isn't of the usual construction, and adds less to the sound than those previously mentioned. A pretty faithful sound is expected, but for me, they were just too bland. If your system is a little too forward, these could be what you're looking for. Thes have an advantage in that they can be used closer to a wall too.
A very similar option to the Performance 6's is the KEF Reference 203/2. Like the MS's they're front ported, so have the close to the wall advantage, but added to that they have some boundary controls which allow you to tweak the frequency response to suit the room. Even though the cabinets are made of conventional materials, they're constructed in a way that cuts down standing waves drastically, with little or no effect on the resulting sound. The Reference range, like many other high end speakers thrive on effortless power, and the Bryston provides that faultlessly. I've found the two match each other perfectly. If any amp/speaker combination stops me going active, it'll be Bryston/KEF.
Just a thought