never heard the ref 18 , but did get a home loan of the nuforce ref 9 monoblocks for a fortnight
first time i ever heard a class D amp , did not like them at all. very hifi not very musical.
dry and overdamped sounding , bit like trying to make a cow from minced beef
if you want a big set of monoblocks try a pair of sanders magtechs
http://www.sanderssoundsystems.com/products/amplifiers/magtech-monoblock-amp
i use the the stereo version and these can drive any speaker