It's more expensive to integrate 2 channel into a theatre system, than it is to just do 2 systems. The main reason for that is that the HT gear is the bottleneck. You need it out of the way, but you need to use it if you have nothing else. And that's the problem.
Sometimes you have to get creative, or use less common solutions. Start with the bass. You may already be in good shape. Is the lfe input on your sub filtered, or not? If its filtered, you can run a regular line level signal to it and the sub's internal xover will do the job, and that's it. If not, you can use an external xover to get signal to your subs. Doing it this way will allow you to get signal to your MMG's without having the signal go through a processor. That's a really big deal if you want good 2 channel sound. Here's a link to a really nice, low cost xover. Would you be OK with it?
http://www.guitarcenter.com/Behringer/CX3400-Super-X-Pro-Crossover-182464.gc
Sometimes you have to get creative, or use less common solutions. Start with the bass. You may already be in good shape. Is the lfe input on your sub filtered, or not? If its filtered, you can run a regular line level signal to it and the sub's internal xover will do the job, and that's it. If not, you can use an external xover to get signal to your subs. Doing it this way will allow you to get signal to your MMG's without having the signal go through a processor. That's a really big deal if you want good 2 channel sound. Here's a link to a really nice, low cost xover. Would you be OK with it?
http://www.guitarcenter.com/Behringer/CX3400-Super-X-Pro-Crossover-182464.gc