I agree with Mark's 3 points above. In addition…
As for switching the CM10s from a rear HT to front music mode, I think the vendor was suggesting connecting the rear HT preouts on the AVR to the aux input on the RC1570 preamp, so I'm assuming I could use the preamp to select the source (HT, Sonos, BD/CD player) for the floor standing speakers. Would that work?
Not in any way that I can imagine. It doesn't mean it's impossible, only that I don't see it based on the AV receivers I know.
Running the AVR preouts into the RC1570 preamp will deliver the rear channel signals, when you want to hear the signal for the front left and right channels. Download the owners manual for your receiver and read it carefully to see if this can be done.
If the sound sources are connected to the AVR, you will not be able select them through the RC1570 stereo preamp. I believe you need to rethink this whole scenario.
I agree with you regarding the front speakers, and am thinking of getting the CM6 s2 instead of the CM5/PM1. I do have a space constraint (in that only bookshelf speakers are an option for the front), so I hope these will be powerful enough.
Both the CM5 and CM6 have (at least) two problems.
First, they both have a sensitivity rating of 88 dB. That means when fed a test tone driven at 1 watt, a microphone directly in front of the speaker 1 meter away registers 88 dB of sound loudness. Both speakers have the same size 165 mm woofer and the cabinets are similar size, so they should have the same sensitivity. The only difference I can see between them is how the tweeter is mounted. So, using the CM6 instead of the CM5 does not solve the problem. Also, because there is no standard in how speaker sensitivity is reported, I wonder if 88 dB is too optimistic for smaller 2-way speakers. In my experience, 84-86 dB is more what I expect.
Second, both speakers have a bass vent mounted on the rear of the cabinet. If you place them in an enclosed space like on a built-in bookshelf, they will sound wrong. They will lack bass, and, independent of the vent issue, the all-important mid range will also sound different because the speaker is so close to the wall. Most speakers are designed to be located in a room "somewhat" distant from walls behind them and to the sides. Of course, how far varies with each owner and room, but moving them further away from reflecting surfaces, like walls, usually creates a more clear sound with better imaging of musicians or singers.
The PM1 does have the vent in front, but they are less sensitive at 84 dB. And, they will still suffer from being too close to the wall, as I described above for the CM5 or 6.
A third problem you may (or may not) have comes because these speakers have the so-called famous B&W yellow Kevlar mid woofer. These drivers have a noticeably characteristic sound that comes from a high-frequency resonance (aka noise) they produce. Because B&W chooses to cross these mid woofers over to the tweeter at an unusually high frequency of 4000 Hz, you can often hear this noise, even though its higher than the crossover point. It depends on the music selection, and how loud you listen. Some describe this sound as "unusually detailed" while others call it "edgy or harsh".
If you use either the CM5 or CM6 and listen at a distance of 16-17 feet, you will have to turn up the volume to achieve normal listening levels. That should increase the chances of hearing the Kevlar noise. Some people own B&W speakers and love them, while others object to the high frequency harshness. It does take time for owners to become aware of this problem. I only point this out because the high volumes that your room might require could make this more apparent for you than for others in other rooms.
