There's nothing wrong with plugging your bass directly (with or without pedals) into the mic/line input on the iMac, I do it all the time.
OK it won't give you great studio quality results, but for laying down ideas it does the job with decent results.
I do have a USB mixer to hand that I haven't plugged into the iMac yet, but shall try it next week sometime & compare (I'll post two clips, one thru the mixer & one thru the mic/line input).