What should I upgrade next?

I would replace the speakers. I have noticed a trend in the Audio magazines which have recommended systems, they often have $1000 - $2000 integrated amps driving speakers that cost $3000 - $5000. The speakers (and how they integrate into your room) are still the weakest link in the stereo chain. Maybe this says something about the quality of todays amplifiers too but, I stand by my recommendation.
I would change the wire last.

speakers. you might even take a years budget and buy some really good ones. you will never be able to detect the other chages in your system if the speakers cannot play it.

imo
Jim

I think you should buy a good turntable and some good records. Then, after you have improved the source, continue with upgrades down the chain, in the order of signal travel. I have heard some pretty low end speakers sound quite amazing when they are fed by a great front end. I have also heard some great speakers sound pretty crappy when fed by a poor front end. If you upgrade in the order of signal flow, you will achieve the best result. The source is the most important, as this is where the signal enters the system. At this time there is no better source than a good turntable. That is where you should start, if you are serious about better sound.

I would upgrade pre-am, and CD first. Then, you can tell if your current speakers would work for you. Or, as suggested, sell your pre-amp and amp(s) and get high quality Integrated Amp (Krell, Plinius, Perreaux...if you need high current amp )Speakers should be upgraded last!

Get rid of that Preamp in a hurry... If you are happy with the sonic performance of your speakers...as far as the musical characteristics... then definately get a good preamp. Try an audio research, or if you dont want to break the bank... a good preamp for the money is a b and k
pro 10 mc....about 300.00... spend the rest on a nice dual mono amp... aragon, 8002 or 8008 bb this will only compliment your system.. provided that you want to stay 2 channel. Another nice amp that i have had is the Musical fidelity a3cr... about 1k used.. the classe ca 150 is nice and even the macintosh mc 122.. all nice pieces.
Good luck