I have the same setup and have found the M1s work very well with the 685s. My problem was, I don't have room far behind my sofa to position the 686s, so they had to be very close to the listening position and I was worried they would overwhelm in this position i.e. virtually in line with the ears. On the other hand, the M1s I could position further back due to their smaller size. I did listen to both setups at Sevenoaks before I purchased and, at the correct distance, the 686s were definitely better. However, switching to the M1s, the difference was only slight I thought. Once I positioned the 686s where I would have to put them in my front room, the M1s did then sound better - the 686s just seemed to be too intrusive (and yes we did adjust the levels!).
So I think if you can fit the 686s in, they would be your best bet, but if you can't, the M1s are an excellent compromise and there's hardly anything between the two.