i have never heard the AT 250, so i cant really do a comparison.
as far as the speakers you are looking to get, the VP150 and M60's would be a lot of sound for that space. i aint gonna say its too much sound, but you will definitely have sound to spare. and if the room does open to the rest of the house as you have mentioned, then it might be just right. another option would be the M22's and the VP100 center. i am just taking for granted that you have, or are gonna get, a sub? its a must have for HT in my opinion. you will definitely need one with the M22's.
i would think the HK would have no problems pushing the 6ohm load. that figure is a little deceiving. the axiom speakers are very efficient, so the 6ohm load will not really be a problem at all for the HK.
good luck in your choices.
bigjohn
