Adcom and 3 series Maggies

I drove my 3.3Rs with Adcom 555IIs and Parasound 1200II THX
amps. The Parasounds sounded better to me.
If I had your options I'd try bi-amping with the 5802 driving the ribbon & midrange, the 565 monos on the bass panels. The 5802 is a MOSFET amp, it's smooth on the highs but weaker in the bass than the 565s.
You have the option of attenuating the Magnepan ribbon, with a resistor, if the 5802 is too hot (loud) on the ribbon.

I actually have thought about bi-amping in that config. there's an orig. XO-1 crossover (orig. equ. with the MGIIIs) for sale on e-Bay right now that I've been eyeing. I'll probably wait till I get a couple of hundred more hours on the panels as they were just completely rebuilt by Magnepan. Still a tad congested in the middle, but improving nicely by the day.