I have had experience with the Bryston 7BST on my B&W Matrix 800's in my second system and have tried them on my JMLab Utopias, which used to have Pass X600's. The 7BST's are an excellent match to any speaker that needs a lot of current to control and drive the bass, which any of the large B&W series really needs to keep them from getting "muddy". Overall, having to chose between the Pass X600's and the Bryston 7BST, I would keep the 7BST. They give up very, very little to the Pass X600's in the highs, nothing in the mid and kill the X600's in the bass and overall response. I am switching to Wilson Benesch Chimera's and will probably use the 7BST's on them, unless I switch to tubes. I agree with the comments that the difference between the 7BST and 7BSST is hardly perceptible.
I would also comment for guys getting into B&W to buy the older Matrix series as they are better speakers then the Nautilus series. Since B&W got bought out, they have gone to more mass market, then to keeping their previous quality and sound. A pair of Matrix 800's or 801 series 3 is a much cleaner speaker (in the B&W series) then anything they have new. Look at all the Nautilus series that are for sale and notice that the price on the Nautilus 801's are dropping, there is a reason. Good luck
I would also comment for guys getting into B&W to buy the older Matrix series as they are better speakers then the Nautilus series. Since B&W got bought out, they have gone to more mass market, then to keeping their previous quality and sound. A pair of Matrix 800's or 801 series 3 is a much cleaner speaker (in the B&W series) then anything they have new. Look at all the Nautilus series that are for sale and notice that the price on the Nautilus 801's are dropping, there is a reason. Good luck