B/W 702 s3

Dramatic difference -- probably not.  Loudspeaker and listening positioning can make large differences in your sound quality.  I would exhaust those options before changing any equipment.

I don't think the Rotel is a good match to the B&W's from my experience with the matching Preamp, I do not have the amp. Little lean sounding and a little bright, not overly musical either. I found the preamp to be neutral leaning to the lean, bright side, if the amp has similar sound, then I would look elsewhere for a quality amp. I would not bother with the tri-bi wiring or amping get a solid single amp to drive them. If money isn’t an issue do mono blocks. 

Adding a subwoofer should make a big difference. They are lacking low end punch. Bi-amp/bi-wire won't do anything to help the lack of bottom end.