As Selden said above, (which I did not know), AVR's wil shut down if they are stressed too much. I only recommend going with an amp because the speakers you are looking at dip all the way down to 2.9ohms. Now granted that is not their nominal rating and it may be only a certain frequency that this happens, but that is pretty low. Unless AVR's have really improved since the last time I had one, that would be pushing it to the limit I think. I personally have 2 amps. One for my front two speakers and one for my surrounds and center.
The 926 have a reasonable sensitivity at 91.5 db. But again, I personally think unless the AVR has a beefy power supply, you may be pushing it to the limit. Both the Marantz and Denon would probably sound fine with Focal speakers. They are both good companies with fine reputations. But again, if it were me, I would be cautious. Just look at Selen's system. If I am not mistaken his system has a Marantz AVR that is probably rated for 6ohm , but his speakers, which are bookshelf definitive technology speakers(Pro-Monitor 1000) are rated at 91db and are 8ohm friendly. So his speakers are easy to drive with an AVR.(Side note, I used to have a def tech system and I loved it
). I would just be worried that your speakers go all the way down to below 3ohms. I may be wrong, and I freely admit that I could be, but that to me seems like it would be putting a lot of strain on just an AVR.
If you can get Denon or Marantz at a good deal, I would probably suggest going with an affordable AVR that meets you needs spec wise, then go for an external amp from one of those two companies. If you do that, you may be able to save money by going with an AVR that is lower powered, but still has all the features you want, and then still get an external amp. Again, these are just my thoughts. If you want to go with the what the store said, that's fine also. But personally, I would not do that.
Hope that helps and does not make it more confusing for you.