Hi,
Yes, it's a possible explanation for slight differences you might have heard. But if the sales guy didn't exactly equalize the volume levels, and one receiver was slightly louder than the other (as little as 2 dB), you might interpret that as sounding "more detailed".
In double-blind tests, slight volume differences have been shown to change listener's preferences, the louder one always being judged as "better" sounding.
You'd be fine choosing the Onkyo for any Axiom speaker except the M80s, unless you thought you might want to upgrade in the future to a pair of 4-ohm speakers like the M80s.
Regards,
