Thanks Arny and A9X. Take a look at this for a real life example of just these effects, to contextualize this tangent discussion:
That is a snapshot of a fool's-biamped horn/direct radiator speaker, untreated room, one channel, from about 2 meters. SS on the woofs, SET on the horns. Note the linearity and distortion. In the SET's band, linearity is not bad at all (impedance ranges from ~6 to over 30 ohms, and the relative peak at 1500Hz is indeed audible, producing a slightly forward sound in the upper mids/treble-that proverbial SET midrange presence). The distortion is kind of ridiculous, although primarily 2nd and 3rd harmonics. Down low, distortion measurements are pretty low, but all the damping factor of the ss amp and linear electrical response is futile in the face of room modes. It does smooth out with both channels driven and bass traps.
Now the SETs trying to drive the woofers does indeed result in about a 6 db suckout, for the reasons Arny and A9X menitoned. I know impedance dips into the 3 ohm range down there, and that's getting close to the output impedance of the amp. Sorry, I didn't save a snapshot of that, but just imagine a 6 db hole from 250-400 hz.
I hope this info helps any lurkers. It is meant purely to elucidate a point, and is not an endorsement of any particular approach.