The effects send makes an excellent tuner output. This allows you to have a silent tuning mode while on stage.  
 
If you're using the effects loop for effects, you have two options:  
 
1. Use a Y-cord from the effect send and feed one to the tuner and the other to the input on your effect.
 
2. Use the low-pass output as a tuner output. Make sure you set the crossover setting at the highest setting so you don't filter out the sounds your tuner needs to hear. Plus, this will only work as long as the volume is turned to greater than 0 on the preamp. You won't have access to a silent stage tuning with the lowpass output.