Bi Amp B&W 803s with 9BSST and 4BST

I am planning to bi-amp my B&W 803 Nautilus speakers with my 4B-ST and 9B-SST ( I have 2 channels available ). Which amp should I use to power the bass / mid-treble? I currently use my 4B-ST to bi-wire my 803s. Should I even consider bi-amping? Thanks for the advice.

Other information: Using Bryston SP 1.7 as my pre-amp and Transparent Speaker cables.