The pre/powers are quite different to their integrated amplifiers, and the XP's are an improvement over the old VS series.
I'm not talking all out power here as this is related to volume, although this applies too. I'm talking current, which is more about drive. Speakers that are harder to drive need more current. Imagine a speaker has to reproduce a snare drum. A well recorded snare drum is very dynamic, and requires a speaker to be very responsive. An easy speaker to drive won't need as much power as a less sensitive one to reproduce that snare drum to the same level. Oh, and the amp needs enough headroom too - I best mention that before the wolves close in.
My point is that for the SCM's to sound as well as they've been designed to, a 40 watt amp that is probably no more than 10 amps or so just isn't going to cut it. It'll sound fine, and to many they'll still sound great, but bear in mind these are designed to work with their own amplification which are 150wpc minimum.