Most important vinyl component to upgrade first ?

I take a very different tack, that of a music lover, not an audiophile, and that is to upgrade your collection of software before anything else.

That said, the cartridge/tonearm interface is really an area of black magic and it is worth doing whatever is possible to find a sympathetic combination. If the best arm and the best cartridge won't play nicely together the result will be utter crap. Throw as much money as you want at the problem and it will not help.

All things being equal, and although the distortions that they generate are a magnatude less than the distortions in transducers, I find that electronic distortions are the most jarring to the ear and most destructive to the fabric of the music. So, if the system was competely balanced, the tonearm/cartridge interface maximized, I would look to the phono preamp.

Happy listening.

I vote for #1, not that you put the darn thing up for a vote. You haven't even tried the, most excellent, SMEV yet. Get it set up and just make a bargain with yourself that you will work to optimize the setup and not buy any more audio for some period of time, 6 months, a year, whatever. In that time commit to listening less critically, enjoying your music collection and challenging yourself by going out and buying some LPs that are out of your preferred genres. Have a glass of wine. You have an awesome rig. Enjoy it, you earned it!