Should I upgrade my pre-amp?

Bryston preamps are always available right here very inexpensivly, for all but the current models. Its hard to think of a component under warranty as aged. In fact its hard to think of a component under warranty, never mind tranferrable for 20 years.

You might also check out the Counterpoint SA 5.1. You should be able to find this in the range you are looking and the former owner and designer (Michael Elliot) is still around to work on them if you would want or need this in the future. I also liked some of the Aragon pre-amps if you prefer solid state equipment.

MFA Magus or Counterpoint SA-3, or SA-3000. Great phono stages and very nice tube preamps. Used, of course, they are both out of business, but can be serviced by ex-factory personnel and designers. If you want to go with ARC, I prefer the earlier models like SP-6.

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)

You could find a used Audio REsearch LS2 at around $500 range that would sound excellent, especially if you upgrade tubes some time. It would be much better.
If you wanted to do the ultra clear "passive" route, you could find an inexpensive passive preamp for pure sonic tranparancy. McCormack TLC's are probably around $400 used or better. Pass Labs Aleph L's are around $600 used.
At any rate, they are very transparant, but less dynamic slam if you do rock or heavy dynamic sutf. Still, the sound is really clear.
I would probably save for a deal on an LS2 or LS7 or whatever, and play with the Tube pre. it's got great overall potential for what it does, and not easily beaten for the price.
Good luck