SEEKING ADVICE: Which component would you upgrade first?

Man what a cool trip back to the day!  

I wouldn't sell any of that stuff. Keep it because it's your first love. 

Now, If you want to pursue HiFi get in the mindset that you'll spend about 5K to get something you'll really love and be proud of.  

Speakers come first. Spend your bucks on a good set of quality used speakers. Magnepan LRS direct from Magnepan is a fantastic honest to goodness audiophile speaker at 1,000.  Get an REL 7i sub for a grand and your have a fantastic speaker set. 

The next thing you will need to spend money on is a DAC. "Put your money in a DAC" as they say (who is they anyway?). PS Audio had  Direct Stream Perfect Wave Mark 1 that they offered $3500 for as a trade in on their Mark II.  Guess what happened? The used Mark 1s dropped to $1800 because everybody got the Mark !!.  I have both- the Mark 1 is 90% as good as the Mark2 and they are fantastic DACs  PS Audio makes sweet little streamer called the AirLens that is a steal. You will pay major dollars (up to 20K)for a fantastic Aurender but it will only be 20% better than the AirLens. 

Next stop is an amp.  I'd be all over a used Audio Research I-50 tube integrated amp. For 1K they will throw in a DAC card and a Phono stage.  Actually you could skip the external DAC above described and cruise out with the loaded I-50.  Used they are +/-4K.  

$ Value Conclusion:  ARC I-50 integrated amp with DAC and Phono card plus a set of LRS maggies and a REL 7 you have about $5500 in a kick-donkey legit HiFi system you will love for many years.  You haven't lived until you spin your favorite records while looking at the tubes glow and feeling the radiant heat from them. It's magic. 

everyone,

long term advice is great, however OP's budget is being ignored.

"for my budget (+/- a few hundo)."

@lukasread you seem to enjoy your vintage system, so (esp. given your budget) i'd stick with vintagey gear. I don't think i've heard the sx-680, but i owned the sx-780 and it was a really good sounding piece. also consider a kyocera receiver, which are excellent and inexplicably cheap in the secondary market i've also owned the hpm100s, which were fun but inessential--i'd look at trading them for something like polk monitor 10b or ads--the hpm100s seem to selling for a grand on ebay, so you can finance some new toys.

@elliottbnewcombjr half the comments as usual have nothing to do with the OP's question

A few options from someone who has been down your road recently. Not sure I would say any of these is the "right" way to go, but any of them will make an audible difference.

One, I don't have any experience with your receiver, but if you're thinking about upgrading your amp the NAD C316BEE is a great option. It brings that classic NAD class AB sound, and has a really good MM phono stage.

Two, I'll echo the previous post about upgrading your phono cart. A big improvement can be had by upgrading/updating to one of the Sumiko oyster carts, the Nagaoka MP series, or the AT VM that was previously named.

And third, if vinyl is your primary media, think about a record cleaner. I use the Record Doctor, which made a big difference in audible quality of my vinyl for not a huge investment. And if you play a lot of used vinyl, an inexpensive ultrasonic cleaner makes a remarkable difference. 

Good luck and enjoy the music!