Let me first say that prior to retiring, I worked 25 years in the audio industry, designing and selling systems for stereo or home theater, with B&W being one of the brands used.
The CM1 was actually introduced prior to the 705, along with a CM2 which was very similar to the current CM5. These never seem to become very popular with the CM2 being discontinued. I will say that I do not know if there is any difference between the current CM1 and the original, although appearance is the same.
I do agree that speakers and amplifier should be matched for the best performance, and yes, the upper models from B&W deserve the best in electronics. Also, I would only recommend a bookshelf on the current AVR, and possibly with the upgraded one as well. These will be less demanding on the amplifier, especially being able to use the crossover to remove low bass. Its true that using the crossover on the tower will have the same result, but that defeats the purpose for using a tower.
So, what speaker? We have always been told that a Home Theater system should be timbre matched. Well, an absolute perfect match is to have 5 or 7 identical speakers. I proved this many times over the years with systems I designed. Therefore, since basically an 805 is being used for the center channel, the fronts should be 805s as well. These would need to be the original, called Nautilus 805, and not 805S, the new 805D, or the older Matrix 805. Now, it is true that the 800 series can be ruthless in revealing the electronics in front of them and the 705 is more forgiving, but the matching center for 705 is a HTM7.
Doing some research on the Marantz SR7005, the 805s and HTM2 should work OK, however, as I said, these speakers deserve better. You can always try the combination, and should it not be to your liking, then upgrade the electronics again, or drop back on the speakers to the new 600 series. They are much better than your current 600s. The B&W 800 series of speakers are usually easy to sell.
In reference to the LCR600, I did a system years ago using 5 of these with an AVR equal to the SR7005 at that time. Also, 2 subwoofers were used since that will help eliminate standing waves. The system sounded much better than many systems that were quite a bit more expensive.
Good Luck with your decision.