Using my 2 channel system with a HT AV reciever.

I currently have a 2 channel system consisting of B&W 805S speakers, and the Cambridge Audio 840E/840W preamp/amp combo.

The 840E has a passthrough ability which allows you to set a fixed input gain for one of the inputs, then run them to a preamp out on an AV receiver. Then you just connect the other speakers to the AV receiver.

My question is can I use an AV receiver that has a different wattage than my 2 channel receiver, or will it sound funny? Also should I stick with the same brand for the AV receiver? (in this case the Cambridge Audio 640R) It didn't receive the best of reviews. (compared to a lot of their other stuff, so I wouldn't mind being able to explore other options, but I have a feeling my center channel might be differently colored in the sound from the mains. (This will probably still happen even using the same brand though)
It won't sound funny but there may be a tonal difference between the L/R and the other channels. That is the price you pay for keeping your 2 channel preamp/amp. The only solution is to have the same electronics for every channel.

It's perfectly fine. You may want to adjust the levels of the speakers if the wattage being fed to the different speakers is materially different (which you should easily be able to do through the AVR). But, in reality, you'll very likely want to do that anyway. For years, in exactly that setup, my surrounds were seeing 105wpc from the AVR, the center 800 from a bridged monoblock, and the mains each 250. The speakers are all different anyway, so it's not like I could make them sound identical even if I cared to try. So I don't. Not a problem at all.