My deck (Eclipse 8053) has biamp outputs - provided the "rear" channels are
switched to become "mid" and the front channels become the "high" outputs.

So, in biamp mode I have no rear channels---but the deck also has a "Fixed
Out" that runs at full volume only.

I ran this fixed out to an old alpine 10-band that I am using to control the
rear speakers. Fuzztone. On the EQ, there is an led array that shows total
overload unless I lower all the bands to minus 12 db (or -15 db, maybe--I
forget) and it's still slightly distorted at those settings.

My plan is to try a 10k resistor (guessing on the value here) inline with
the tip of the interconnect rca before the EQ.

Is this plan valid???

Another idea, better, would be to able to sum the mids & highs for the back
speakers *without having them be summed to the front speakers. That way,
fronts and rear would be controlled by the head unit's volume control. I
don't know how to build this circuit, other than buying a little mixer.