I seem to differ in my opinion from most of the other posters. I personally think you're better off getting the power amplifier before the speakers. This is partly because you already have the preamp, but more importantly, you may damage the new speakers if you try to power them with a severely under-rated amplifier. I assume that if you get new speakers you'll be driving them with something rated for your current speakers, and when you turn up the volume you'll likely be clipping it.
Better to accidentally blow the current speakers than the new ones.