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.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated