As the SL-2 only has a global selector switch for input level- your best bet is to boost the turntable signal up to line level.
You can do this by getting a phono preamp and connecting it between the turntable and SL-2.
The turntable is then feeding line signal into the SL-2 and should work fine.

Using a phono/line attenuator will help boost your phono to line level or bring your line level down to phono. We recommend the first scenario. There are many makes and models out there, here is one that I have seen used - www.mcmelectronics.com