Well basically, I uses my fx loop in a straight signal chain, which is, I put my preamp into fx and then send them into power amp for amp use, AND directly to mixer (using cab sim). I'm pretty sure what outputs that I wanted and where it should go. Just looking for device that match with my signal chain, to make sure it has no overload signal.
And this what I've been talking about with @JayGee, about headroom, to be precise. My preamp max output is +8dBu and max input for Boss DD 500 stated +dBu, I never went crazy about the output, and halfway does already loud for me. So, maybe it'll be about +4 or +5 dBu into the DD 500, and still have about +4 or +3dBu headroom, which is not much, but enough I guess. That's why I concern about how it handle line level signal.
Thanks anyway, really good perspective for me to consider.