Hello John,
Your instinct about series connection is correct: Don't do it if you want smooth frequency response, as series connection of multiple speaker pairs causes the impedance curves of the connected speakers to interact with eac other, which may cause peculiar response anomalies.
I have anecdotal feedback on the Pioneer 1015 with 4-ohm loads and it seems that so far it's OK with the 4-ohm M80s, as well as some insensitive 4-ohm Dynaudio speakers that a reviewer friend of mine used to review the Pioneer.
At Axiom, we also have a number of customers who are using the Pioneer 1015 with the M80s with no problems of current-limiting, overheating or protection-circuitry shut-down.
Regards,
