Poor sound from Rega Planar 3

Oh, one more thing that could be causing the problem you describe:

The cartridge you have may be a low output moving coil (you said you weren't sure which model it is). If you try to play a moving coil into a standard phono preamp you will have very little HF response. Do you need to turn the volume up a bit more than usual? If so then it may be a moving coil cartridge. If you have a spare moving magnet cartridge (most are) you can test this by mounting it on the Rega and playing it, or even just hooking up any random turntable that has a MM cartridge to your system.

I'd second the recommendation of a new Audio technica at-95e if you want a low cost high performance cartridge. It does sound like your cartridge is worn. the black stuff you see on the tip could be vinyl. If you do get an AT95 get and like it get a couple of spare styli now as I think they're being discontinued. When installing the cartridge pay particular attention to overhang and alignment as failure to do so will destroy imaging (as I have learned myself).

Personally I'd chuck the cartridge since LPs are hard to replace, but you could try giving it a little swab with a cotton bud (Q-tip) soaked in isopropyl alcohol. I've always found it to work ok with no negative effects.

Also clean the belt in hot soapy water so it feels sticky and clean the motor shaft and the sub-platter with iso-alcohol so that there's no grease to allow the belt to slip. Finally a little trick I use on my rega is to use double sided tape to hold the felt mat to the glass platter to prevent slippage. Speed stability is not the rega's strongest point but if everything is clean and the mat taped down then it helps a lot.

If you are still following this thread I should also mention that when we used the AT120e the original Rega felt mat worked well with it (it's a plain black thick felt mat), but when we tried a Linn mat for whatever reason it sounded thin. Our overall experience with turntable experiments was that the compatibility between the various parts (cart, arm, mat, etc.) is just as important than the parts themselves. Some trial and error may be neccesary.