Looking purely at specifications for both, the 601-S2 has a frequency response starting at 70 hz vs the 686 at 55 hz. So the 686 will produce more bass extension and go lower. The 686 has a sensitivity of only 84dB vs 88dB on the 601-S2. This means the 686 is not as efficient and will need more power to drive it but it incorporates all of the latest technologies and features from the new 600 series B&W loudspeakers. Thes bass/mid-range driver is physically closer to the tweeter which improves integration & vertical dispersion between the two. The bullet in the center of the driver is now fixed vs moves which smoothes the response in upper frequencies. The material that surrounds the tweeter is new improving spaciousness and imaging. Yes, the 686 is an amazing loudspeaker. So to answer your question, you would definitely trading up by selling the 601-S2 series and going with the 686. Just my opinion. Read up on the 686 at the B&W main website.
