The second of the two preferred IF at all possible (i.e., if your source can "drive" two amps, the main & the sub). If not, go the other way.
You'll be bale to tell if it's working through the low frequency energy -- if there's little ("no bass") then you;re not driving both amps in parallel.
You'll be bale to tell if it's working through the low frequency energy -- if there's little ("no bass") then you;re not driving both amps in parallel.