Help me with a system upgrade

Hi all,
I've been hanging out here off and on for several years. And my knowledge has benefited greatly. I've fortunately come in to about $500 to put into upgrading my system. Where would you suggest I put that money? It's fun to spend other's money, right?

My System-
ipod (all lossless) and Kenwood KD-2000 Turntable (unknown cartridge)
Adcom GTP 400 (a tuner/preamp is very practical)
LM3886 Chip Amp (BrianGT's design)
Cheap interconnects and DIY extension cord cables
Speakers are DIY Seas Monitor 5" (Zaph's L15 and 27tffc design)
Dayton 12" powered sub

-My listening/living room is about 12'x12' and the sound spills over into the dining room and kitchen.
-I have no interest in home theater.
-Aesthetics and simplicity matter. The fewer components the better. No large open baffles. A slim tower might be possible.
-I listen to all types of music, but I really LISTEN to jazz.
-I do listen to the radio a lot, so that has to fit in somewhere.

So where's the weakest link and the most cost effective upgrade? The speakers seem like the most competent components. The sources are unlikely to change, though a different digital source or t-table upgrade are not out of the question. I've thought very seriously about buying a yaqin tube amp and a cheap tuner. That way, I could ditch the adcom (the most outdated piece). And I have a thing for tubes. I'm not really looking to DIY anything at the moment. This is more of a purchase, remove from box, plug in kind of upgrade. But something I could fiddle with in the future would be great.
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I should add two things.

-I'm happy to lurk around for used things on ebay, audiogon, or craigslist.

-If I were to assemble my dream system, it would probably be a Linn turntable and ipod, Jolida or similar tube amp, cheap tuner, and the Zaph ZRT towers. These items choices are really informed by aesthetics and functionality, rather than real world listening. Shallow, I know.