im thinking of buying a dj mixer soon and i was wondering if having that setup with the sp in a send/return loop allows you to play a song on one channel and play the same song with effects on the other channel...

yeah this definitely works
i've quite often used it as an effects box with the send/return on my mixer

it's great fun

It's going to depend on if the mixer has a send/return loop. A lot of DJ Mixers don't (scratch mixers in particular are usually missing this feature), I'm sure more of the House and 3 channel mixers have return loops.

I don't have a mixer with send/return, so I use the 404 as the final output for my live rig; that way I can apply effects to the entire output of my mixer (Turntables, CDJ, other random noisemakers). Then I have samples and beats that play off the 404 that are (sometimes) un-"effected". You could use the 404 as an intercept between the table and the mixer, and apply FX to only one table that way. I've always considered doing that with my extra 303, but I just don't have room in my rig, nor do enough situations arise where it's worth it to go through the trouble.

I've got my 404 hooked up to my Vestax PMC-05 ProIII

- it's a solid way to sample on the fly from vinyl, merge your beats and add effects to 1 or both of the channels. for now my setup is quite limited though, as the only worthy real time ext effects are things like delay and echo. things like a tremolo effect are pointless as the effect and the original source get outputted at the same time - same with distortion and other effects that radically change the sound. of course, setup the 404 on your main mix output and you can use every effect to great advantage, but lose on specifying channels.
right now i've got a korg kaoss 3 processing my main out.

i've tried that and it works ok, except when i spend 15 minutes trying to figure out why no sound is coming out of my tt only to realize the 404 is off. my mixer actually has a send/return loop but as the documentation that came with it was pretty sparse i've never been quite sure how to use it.

what are the send/return outputs of your mixer? you should be able to 'thread' a line out from the mixer through the 404 back to the mixer. then you can assign x amount of the 404 effect to your mixer channels

alot of the 404 effects do have Balance though, which can do the same thing. i usually just use Balance on flange/phaser,etc effects when i want to spice up a signal

I've always wondered if that was possible! Is there a danger of creating a feedback loop, though?

yeah i would be wondering about the feedback think too

yea you know I've tried hooking up my mixer to the 404 a few times and after fiddling with it for a while I realized I don't have that send/return loop feature. Instead I have to turn on the headphone sound of the channel the 404 is on and turn on the EXT source on the 404. Then I get some feedback of effects applying to my tables, but it sounds horrible and will only work if the headphone sound is on....so strange and I never understood why this happens.