Greg, why is mono-amping the most ruinous solutionI misunderstood this to mean, use MONO blocs all around... sorry!
I would agree however that passively biamping is expensive and only sometimes beneficial -- biwiring likewise.
As you imply, it's best to get a grip on the speakers first and see what can be done later.
Methinks I got carried away... pardon! It's clear that with the lowest crossing set at 800Hz, it's going to be very difficult to devise an active solution: the ear is still sensitive there.
Now, if you had another cross at say 100-80Hz...
Cheers