Okay, from this late follow-up, you can see that I take forever to make a decision!

I've decided that I'm going to upgrade my speakers piece-meal, replacing the l+r fronts and the centre first, then replace the sub and rears later. I'm going to get the M3's and the VP100. My concern is these speakers will be used in conjunction with the Yamaha speakers that came with my HTiB. They are 6 Ohm and the Axiom's are 8 Ohm. Is this a concern? The receiver (RXV459) can be set to output either 6 or 8 Ohms. Is this advisable?