can a denon AVR-891 run a CM9's ?

At certain levels you will be ok but don't expect to be able to crank them really loud, especially without a sub. Those CM9s have some impedance dips down to 3 ohms that a lot of receiver will have a hard time standing up to at high volumes. Also the 891 has no preamp outputs so you would have to get a different receiver to add an amp. I think I would look at adding a second sub and keep the smaller CMs unless you find some more room in the budget for a receiver/amp upgrade.

I like b&w, but compared to other brands that sound good for less, b&w is a little over priced. You don;t want the receiver to go into protection mode with the dipping that mjpierce is talking about at high volumes. I don't think the upgrade is worth it.

In practice, it wouldn't matter, since the difference between 130WPC and 170WPC is negligible (just a tad over 1 dB perceived volume difference, which is basically inaudible).

I know that the CM7s have a 5dB or so dip (right in the mids, if I remember), which is kind of large and it may be audible. Not sure the CM9s have a similar dip, but even if they did, power by itself wouldn't do much for it -- room correction may be able to compensate for it.

Either way, these are rather sensitive speakers (I doubt the momentary impedance dips would matter much in daily use) and in an average room you are unlikely to play it so loud as to experience any issues:

With speakers of 89dB sensitivity you need 100WPC to achieve just over 102dB SPL at 10 feet.

Even an hour of this sound level a day will cause irreversible hearing damage rather soon, which I suppose would mean that you'll then be looking either for a much more powerful amplification, or for a hearing aid.

The 4311CI is designed to run 4-ohm speakers at reference level (0db) volume whereas the 891 is not, so at reference level listening the 891 is more likely to shut down in protection mode. As mjpearce suggested, if you only listen at average volume levels (eg. -20db), then the 891 shouldn't have any issues.