Pre-amp out splitter won't power subwoofers? it does power DSP unit.

Any thoughts are appreciated. I'm using a pre-amp output from my STI-1000 integrated, into a DSP room correction unit, and then back into the STI-1000 via HT Bypass inputs.  I have several subs but so far the only method that powers them with a line-level signal is using a Y splitter coming from the DSP unit's analog line level outputs. In this mode I cannot use a high-pass filter on my mains to relieve them of bass 60 and under +/- as it feeds the subs as well.

Using a Y out block connector from the STI-1000 pre-out (male/2 fem) when I run a cable to the DSP it works (and back to STI), but the subs will not work being fed a signal in this mode. There is no discernible cone movement or vibration. It seems strange as the DSP got its 'half' of the Y split's signal. Maybe the sub needs more to even activate?

Coming out of the DSP output works, but it makes the mains play very low frequencies unnecessarily since they take full signal (and want to play low). So, what am I doing wrong here?  I can't quite figure this one yet, but it may be simple enough. Thanks, folks.