The VP180 is a 4 ohm speaker. My AVR is rated down to 6 ohms. Denon X4000. I was looking at investing a 4 ohm stable mono block for the VP180 and they are few unless you have lots of money e.g. QSC ISA280. I wonder about a few things 1. can I use the preamp output of the X4000s center channel to a stereo amp and get output from both channels? If so can the VP180 be wired as if it was two speakers?

Your Denon won't have any problems driving your 180. I drove my M80s with a cheap Denon circa 2002 for years. My Onk is now happily driving them. Make sure you set the soft switch on your Denon to 8 Ohms and not 6 otherwise it will reduce the voltage to your speakers and it will sound not as good as it could.

You can't wire it as two speakers nor do you need to. If you really want to go with an external amp, you have pre-outs on that receiver (above the speaker terminals just to the right of the mid-line). Just stick an RCA jack into the center pre-out and take that to an amp. Bring two wires (+ and -) out of the amp and wire them to the 180.

Andre, I had the exact same concern when I ordered my VP180 to be powered by a 6 ohm rated receiver (Onkyo).

The real issue you can face is taxing the receiver which will cause it to overheat. To put my mind at ease, I ordered an Aircom T9 https://www.amazon.ca/gp/product/B01LW1Q4X9/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

I can happily report that my AVR runs nice and cool with the Aircom and I've run it pretty hard and loud.

That said, the Denon should be fine without the Aircom but it's nice to have the peace of mind to run that beast of a VP180.