I have a small battery powered desktop Marshall guitar amplifier I sometimes use which is really nice for sending samples through from a Sony Walkman or a cd player into the SP-404 .
I use it mostly with a clean signal and apply any FX to the sample from the 404's internal FX but the Marshall also has a second gain channel which is great for adding extra filth if its needed.I also sometimes use the little amp with a vintage MXR Phase 90 stompbox rigged up to it and have gotten some really nice sounding effected samples that way.
Those Marshall amps are cheap as chips btw, the last one I saw was only £10 so definitely an extra bit of kit anyone can afford to buy and pretty useful too.
Oh yeah, I almost forgot to mention that you can actually play a guitar through it too !

Heres a demo I made last week which uses the Marshall to process the chopped up vocal and guitar samples in the track if anyone wants to listen.
https://soundcloud.com/user-58305257/pl ... h-dynamite