Ten, welcome. My suggestion would be to disregard the advice of any amplifier or speaker manufacturer representative who suggests that certain amplifiers and speakers form an incompatible combination. Any amplifier can drive any speaker, regardless of what impedance rating happens to have been applied to that speaker. The question is how loudly it can be driven and/or for how long a period without shutting down. The Yamaha and Onkyo receivers that you mention have driven 4 ohm loads more difficult than the M80s(a speaker with slightly above average sensitivity which requires less power and current that some other speakers, including some rated at 8 ohms)quite satisfactorily. Real world listening often differs from extreme lab tests, and just about any amplifier can be forced to shut down if driven long and hard enough. Don't assume a problem which rarely develops.
