SP-404SX/Mixer line-in setup question

[Lord_Maitreya]

I just got an SP-404sx and I am trying to set up a line-in configuration with my computer from the mixer, but I've never used an RCA hookup before. I have a four-channel mixer with tape in/tape out, to which I have hooked up the line out/line in respectively (on the SP). I have the main out from my audiophile sound card hooked into Channel 1 on the mixer, and the main out going back to the input on the sound card, and I am using the headphone out to my monitors. It's a roundabout setup, but I've used it for years with my Electribe and it's done me just fine; however, as I suspected, when using the "EXT SOURCE" function on the SP, I get a kind of flange effect on the audio (most likely due to the audio being routed through the mixer twice). Should I hook my sound card directly into the Line In on the SP and the Line out to the mixer so all audio passes through the SP? Or is there some other configuration I should consider for line-in recording from the mixer? Sorry if this seems overly basic.

Thanks in advance, everyone.