Anyone ever use a Dynaco Quadadapter QDP-i

This is known as the Hafler circuit, and there are many articles about the circuit, and the man, David Hafler, available to ponder.

I owned and used one when they were new- maybe 1977 or thereabouts. I recall from the user manual the recommendation that all 4 speakers have similar efficiency and impedance ratings. And- the powering amplifier must have (or use) a common ground between channels. Most do, but it is important to verify!

I really enjoyed my time using the qd-1, especially with certain albums. At the time, I was using Dynaco speakers. I may still have the original owners guide…

If you don't like the effect you could call it diminished.A

It is a very simple circuit.  Basically you run a wire off of each channels positive output and run it through one or two rear speakers (in series) and throw on a volume control.   You need the rear channels placed as far back as you can to add some delay.  Used it once 20 yrs ago for "home theater" off of my two channel system.  I was quite surprised at how natural it sounded if you can set it up right.  I would guess the added demand on the amp is minimal.