In my limited experience, when you can get "decent" output out of your bass guitar and recorded music, but the kick just really seems to be lacking something AND the subs are underpowered (as in this case) it's seems to go back to needing more amp behind things to really drive them...and the big amp not bridged always seems to do it better than the smaller one bridged.
The folks who spec'd and installed our house PA back in the day put a pair of SB1000z's on a MacroTech 3600VZ in stereo and it really seams to fall into that boat as well at 1565 watts a box.
They seem underpowered to me as well. Maybe you could borrow, rent something to try out some options.