Keep in mind you will need a pre-with a phono stage, a common and good ARC unit is the SP9 (Mk II, or III) preferably. It is a tube hybrid. (in ARC "land" the SP prefix denotes the presence of a phono stage.) There are a variety of other tube units out there depending on what you want to hear (which is unspecified).... More important is:
Why do you want to upgrade? What do you want to hear that your present system doesn't let you hear (or visa versa)? $4-500. isn't a lot, and I don't know if you will get something better than your Haffler. A more cost effective upgrade path in the "high end" is to consider selling your Nak as well and getting into an integrated amp. This may leave you hanging if you really want separates later, but depending on what you want to hear, there are lots of improvements to be had over your system. (I know, as I still have a Nak Stasis piece in a second system)
Why do you want to upgrade? What do you want to hear that your present system doesn't let you hear (or visa versa)? $4-500. isn't a lot, and I don't know if you will get something better than your Haffler. A more cost effective upgrade path in the "high end" is to consider selling your Nak as well and getting into an integrated amp. This may leave you hanging if you really want separates later, but depending on what you want to hear, there are lots of improvements to be had over your system. (I know, as I still have a Nak Stasis piece in a second system)