Yes, there are a number of manufacturers who make preamplifiers, though most are not in the same price range as the typical receiver. There are a few that are quite reasonable from Internet Direct companies.
I know for a fact that all Marantz receivers state they do not support loads lower than 6 Ohms, but I have run 4 Ohm speakers off even lower models without issue. It all depends on how much volume you expect to get, as 4 Ohm speakers draw a lot more current which means less total headroom for the same SPL.