Passove = You don't mention what cartridge you are currently using. An upgrade here could well give you a major improvement.
A couple of points here before you start spending loads of money.
The SL1500 is a pretty good turntable when set up properly. The speed accuracy and wow & flutter standards will be difficult to match in any modern budget turntable. The tonearm is pretty good as well. It is a typical Technics medium mass arm which will suit the plethora of medium compliance cartridges available. Something like the Audio Technica VM95 series are good value (especially the VM95ML), or maybe a 540ML if budget will allow. There are good cartridges from the likes of Goldring, Grado, Ortofon, Sumiko etc which will go well with your turntable. I think that a well set up decent cartridge will probably give the biggest 'bang for your buck' because if you have a worn, or poorly set up, or very average cartridge at present, no matter what you feed it into you will not make much improvement because you haven't got a decent source in the first place.
Your amplifier has 5 separate power supplies each dedicated to separate systems within the amp, so the power amp section and the preamps are well isolated from each other. O.K. the internal phono preamp is not the best out there, but it isn't that bad either. It will probably be more forgiving than an expensive dedicated external phono preamp until you have a front end that exceeds it's capacity to resolve the fine detail in the signal.
So all I'm trying to do here is to guide you in a sensible budget oriented direction. You can spend megabucks in this hobby and get next to nowhere if you haven't got the basics sorted out. On the other hand, you can do a little research and spend a little, wisely, and get quite stunning results.
Your, turntable, amp and speakers are basically O.K, in that they are decent quality good budget choices. So the way to go is to start at the front and concentrate on the turntable. Get it set up level on a good solid surface and then check the cartridge alignment. Downforce measuring scales and alignment gauges are cheap, and necessary to ensure that your cartridge is correctly aligned. As I said above, from that point I would first consider a decent cartridge (if you haven't already) and then go for an affordable external phono preamp. The next area to upgrade would be the loudspeakers in order to take advantage of your improved signal source, but start by making sure that your existing speakers are positioned and mounted correctly, then possibly upgrade them.
Most important:= Do your research and make 1 change at a time in order to evaluate the changes properly (you can always go back a step if it didn't change as you expected), Make notes of what you have done and do it slowly and steadily.
Good luck. Any queries or advice, obviously just ask the people here on AK.
