I replaced my 500s with Trackmaster II SKs a short while back and I am very happy with them, though I had to spend quite a bit of time experimenting with the correct tonearm weight - you will need to knock this up to at least 3.0... It appears to be considerably cheaper to buy from the US (about 50% of UK costs), but I tried several places all who could not understand some combination of the concepts of credit card payments, international shipping or keeping items in stock. I ended up getting an excellent deal through EBAY (3xcartridges + 8 styli + shipping for £140)
With regards to the DJ mixers, I have the Technics Mixing Championship mixer (can't recall model number) and the sound quality I get with it is considerably worse than if I directly attach the turntable to the amp; I have been told that it is a fact of life that mixers will do this - has anyone got any suggestions here as to anyway I can improve things or recommend any mixer which does retain decent sound quality? Cheers David