Well, I did find this in the M6 users' manual:
WARNING: Always make sure proper load is connected before operating the amplifier. Failure to do so could pose a
shock hazard and may result in damage to the amplifier.
But I found the exact same precaution in the MPulse manual and I know for a fact that amp can be run without a load, so...
I guess it would be better to heed the warnings of the manufacturer, even if it doesn't really make much sense - I even looked at the block diagram of the amps in question and they do not use heavy iron for the output which REALLY makes me wonder why they would require a load - honestly, my belief is that they simply used the same precautions page in most of their amp manuals, but it IS in the manual.
Bottom line - if I had a SS Mesa under warranty, and running it without a load would cause the warranty to be void, I'd think twice - even though common sense tells me otherwise - it's a weird one - I just don't see how a direct coupled output would need a load.
Maybe Jule can offer his insight into this.