+1 on your plan B. If you want to get better sound quality and better performance you should upgrade your speakers and get a good quality stereo integrated amp that has HT bypass feature for your stereo music playbacks and use your Marantz SR7012 av receiver for HT (surround sound movies).
You could upgrade to higher end series B&W such as the new 700 series S2 and these will be big improvements from your 685 S2. If you prefer a stand-mount or bookshelf speakers I highly recommend the 705 S2. They are the sweet spot in the entire B&W speaker line. The 705 S2 have the best price performance ratio in the entire B&W speaker lineup. They only retail for $2400/pair new without the stands but perform very well and come pretty close to that of the 805 D3 bookshelves except the 805 D3 cost $6k/pair new. The 805 D3 are better but not twice or three times better as the price suggested.
If you prefer floorstanders I highly recommend the 702 S2, which is the flagship of the 700 series S2. They retail for $4500/pair new. The new entire 700 series S2 do not use kevlar midrange drivers anymore and use a new material called continuum midrange drivers like the ones used in the 800 series D3 speakers. Both 800 series D3 and new 700 series S2 use the same method in decoupling their midrange drivers from the cabinets. Trickle down technology from the 800 series D3. Sonically the new 700 series S2 are close to that of the smaller 800 series D3 eg 804 D3 & 805 D3 but are priced much lower than the 804 D3 or 805 D3. Sonically the 702 S2 floorstanders come pretty close to that of the 804 D3 but are priced only half of that of the 804 D3 ($9k/pair).
You should have a listen to them. They share much of family resemblance in sonic qualities between the new 700 series S2 and the newer 800 series D3 speakers. They srounded more natural and organic than the previous B&W models, including your 685 S2. They don’t sound anything at all like those previous B&W models including your 685 S2.
If you upgrade to the new 700 series S2 speakers you will have to get their matching center channel speaker as well. If you go with the 702 S2 floorstanders you should get the matching B&W HTML71 S2 center speaker, which is the bigger center speaker for the 700 series S2.
If you go with the 705 S2 bookshelves you can get the smaller HTML72 S2 center speaker. It is very crucial that you get the matching center speaker with your main front LR speakers for timber match. It will also be most ideal if you can also get the matching surround speakers but not mandatory.
As for a stereo integrated amp choice I would recommend the following :
* Parasound Halo Integrated HINT6. Has HT bypass feature and has built-in DAC. So you can utilize its internal DAC. Has USB DAC inputs, spdif (coax digital), Toslinks (optical digital) as well as analog audio inputs. Highly recommended! It will sound good on the B&W 702 S2 or the 705 S2.
* Rotel RA-1592 stereo integrated amp. Has built-in DAC and same inputs as the Parasound HINT6 but the Rotel does not have a dedicated HT bypass input. However, you can configure one of its analog audio input as unity gain input for HT bypass. Just set the volume at 80 (out of 1 to 100) and save the setting for that particular input. So every time you select that input the volume will be disabled and fixed at 80. 80 is usually a reference level for unity gain setting. Unity gain is basically a HT bypass.
This Rotel RA-1592 will pair really well with the B&W 702 S2 or the 705 S2 if you decide to go with the B&W. Rotel and B&W always have great sonic synergy together. B&W, especially the 600 and 700 series, voiced and tuned their speakers using Rotel anplifications and electronics. They are meant to be paired together. But the Parasound Halo HINT6 will also sound great on the B&W 702 S2 or the 705 S2.
* Musical Fidelity M5si or M6si. They both have internal DAC and have USB DAC inputs. These are very refined and musical sounding integrateds. Not sure if these have HT bypass feature or not.
For HT ( surround movies) you can still use your Marantz SR7012 av receiver but if I were you I would upgrade to the new Marantz SR8012 flagship receiver. It performs better than your SR7012. Better bigger power supplies, better analog audio stages, better amp section, better DAC, better preamp processor section, better everything.