There may be a solution to your problem with the subs, but first let me ask you some questions to find out about how you did your connections and how things are wired.
I'm pretty sure that you did some sort of high level tap off of the back speakers.
Which option did you use for getting the signal to the amp;
A) Used a high level input plug that comes with the amp and extended those wires to the back speakers ?
B) Did you use a line level adapter, in which case you would have tapped the rear speaker for signal, and used a part that gave you RCA output into the amp ?
If you used option B you could upgrade to a vairable line level adapter. Both of which should be available online, at BestBuy or the like.
You could also use an Active crossover.
Also if you wanted no Bass at all you could cut the remote wire at the front of the car and put a toggle switch on it this way it would kill power to the amp.
Just a couple of options to consider,