I think Hipnotic is right - if you enable the x-over function, you cannot use the speaker outptus at the same time. In order to use it in a 2.1 config, you'd have to run the sub off the pre-out and the sub would have to be powered and have it's own crossover.
75w @ 4 Ohms means about 40-50 @ 8 Ohms, so it does't sound like this guy will get incredibly loud unless you use it bridged - loud enough for a medium sized room probably. My monoblocks are about the same size as that guy and run 180w @ 4 Ohms, so I'd expect the best use of one of those guys would be as a monoblock in bridged mode, though I don't see it listed as being 4 Ohm stable when bridged. It also doesn't offer any other functionality that a stereo receiver would, such as a remote, additional inputs for other sources, etc...