I tried first a daisy chain monotribe, followed by a boss equalizer GE7, ending with sp404sx without success.
Anyone know what kind of adapter should I use to transform output signal (mono I think!) of any boss pedal, to the input signal (RCA) of my sp404sx?
thanks!!!

Why not just run into to the mic input on the front. I've done it a bunch with guitar. Works fine.

You're right most guitar pedals are mono. Try this into the left OR right input

Make sure your 1/4" cables have one black stripe on them and not two.