XLR cable placement

It depends on sensitivity of specific unit to cable change. Tube gear is typically most sensitive, with older, simpler designs being most sensitive.

Easy! Put it between the DAC and the preamp and listen to it for one week, then move it to the other location and listen to it for another week. If you still can’t make up your mind then the location doesn’t matter. You can’t lose. Good luck and have fun. 

I’ve found putting my best/most resolving interconnect between my DAC and pre to be by far the best way to go, but YMMV so try both ways. 

I might be crazy but I’ve always bought into; you get the sound relative to your worse component and cable in the chair. Based on that premise I would start at the source, DAC to Pre.  I want my Pre to start with the best signal that I can afford to feed.  At some point you will probably change the XLR between your pre and amps.  Not sure if this makes logical sense but it does to me.  I also believe in over investing in speakers if you over invest in an individual component.  Your speaker cables should always be your best cables.  If they aren’t you may not be able hear a component or interconnect change.  Sorry for running on.