The first real item--Why the heck do you have/want such a complicated setup? That is never ideal and invites problems or mistakes in your system.
Next item, when you add 2 amps in PARALLEL, the load seen by the source will NOT add to 115,000ohms! Rather, the total ohms must go down (more difficult load to drive), lower than either of the loads by itself. A simple thought process will make this clear--the electricity now has 2 paths for flow. You need to review the calculation for adding loads (resistors) in PARALLEL, you used the calculation for SERIES loads.
In your case, the input impedance would now be 13,000ohms if you connect both the sub and the amp to the same pre-amp output.