Sorry if I wasn't very clear spaceman. The 3 ohms the speakers drop down to cause problems for most receivers because it is a very difficult and demanding draw on the amplifier section. Only very expensive receivers like the 5805 that Corysmith pointed to have amplifiers that can drop down to those levels without causing major strain on the whole unit. I guess the easiest way to explain it is this:
8ohms pretty easy to drive as long as the speaker is efficient
6ohms not too bad I'd suggest a mid level receiver even if the speakers are efficient
4ohms hard to drive, you are getting into speakers that need some real power here, either expensive reciever ala Denon 5805 or a seperate amp will most likely be needed no matter how efficient the speakers
3ohms murder on most receivers, either
big bucks spent on a receiver or seperate amp needed for sure
2ohms the receiver police show up to arrest you if you are only using an underpowered receiver, these kind of loads require insane amounts of power, most amps capable of pushing these loads effectively have two plugs so you can plug them into two outlets
The lower the ohms generally speaking the harder the amp is working. The harder the amp is working the more heat it puts out. Heat is bad because receivers are full of delicate computer chips. There are jokes about guys frying eggs on thier amps. The 804's won't be dipping down to 3 ohms very often, but just knowing that they can would make me seriously consider getting a seperate 2 channel amp for the front pair. This may sound like I'm being snooty but B & W assumes that if you can lay down the dough for speakers as nice as the 804's then you can lay down the dough for proper amplification.
What you then need to worry about is the capability of the receiver you choose with regards to how well it can integrate a seperate amp or amps. You would be hooking up the amp to the pre outs on the receiver. Pre outs are what you hook the amp up to in order to integrate it into your setup. The Yamaha 1500 and 2500 seem to share the same high quality pre amp section, which one of the reviewers stated was the best in the mid price market. Essentially that means you won't do any better without spending some major dough.
I suggested the Yamaha 1500 because you said you'd like to keep the option of going 7.1 open, but that this is a secondary consideration so the 1500 seemed like a better fit than the 2500, and is a little cheaper. The good news is that the amp on the 1500 will have no trouble at all running all the other speakers in a 7.1 setup so you don't need any other seperate amps, unless you choose to go all 804's all the way around that is
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Hope it helps and sorry if I was too vague before.
