Current production Slant 6Vs do not have an effects loop. This question/answer only applies to Slant 6Vs with the serial numbers 521 and below.
The Slant 6V's effects loop is a simple, unbuffered insertion point between the preamp and power amp sections, and the level of the signal coming from the effects send is dependent on the volume setting of the amp. If you play at high volume, the effects send signal level may be too high for some pedals to handle. In this case, the signal level going back to the effects return may also be too low because the pedals cannot supply the same powerful signal at their output.
One way to correct the problem is to make the first pedal in your loop one that has an input level control. You can use this to turn down the signal to a level that works with all the effects that follow. The last effect in the loop should have an output level control, which can be adjusted to boost the signal back up to where it was before entering the first effect.