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.

Frequency reponse on those speakers is 46hz to 26khz, so, bass and thickness may be lacking to your ears...  Try pushing them closer to the front wall and farther away from the side walls.  This should increase bass and reduce any reflections present.  Also try toeing them in or out, depending on how they are currently set up. 

Note:  Usually for full range towers you want the low range frequency response down to approx. 21hz... so, as others have said, a subwoofer may be the fix.