from B&w 805's to Usher be-718 - what to expect?

That sounds more like a downgrade.

Why is that a downgrade

I've heard both of these speakers and would say they produce a very 'colored' un-natural sound. I suggest you hear some single driver speakers to put balance in your evaluation before making an switch.

Big downgrade.

The 718 are highly overrated.

I had a pair here for a couple weeks; drove them with a pair of highly regarded Class A solid state amps as well as Class D amps.

Chalk on a chalkboard. Much too analytical and clinical for my ears.

Stay with the 805; they are the lesser of 2 evils.

IMO