Since I doubt that the OP has horns installed and it is possible to turn horns down anyway who gives a rip if the subs can't keep up with a set of horns. Horns are the anomoly, far from commonplace anyway.
For SQ focus on the front stage. Since you are happy for now with how it's set up fill in the bottom end. In all honesty if the front stage is doing its job pretty much any sub will be up to the task. It doesn't have to be some mythical SQ sub just properly enclose it and give it an amp. The amp doesn't even need to be all that powerful.
there you go... subs can't keep up with 4 id xs69's with over a thousand watts going to the mids. horns aren't the issue.
