I am sorry that I misread your original where you said that you wanted to "keep the full range signal from my main speakers." I read "to my main speakers."
So, the use of a dedicated sub crossover would be what you want and the P2100 would do that, albeit without the valuable EQ.
There used to be good dedicated sub crossovers on the market but they have disappeared. If you can find an NHT somewhere, they were great and cheap enough to let you add an AntiMode 8033 for EQ. I also recommend adding the 8033 to the P2100, if you can.
So, the use of a dedicated sub crossover would be what you want and the P2100 would do that, albeit without the valuable EQ.
There used to be good dedicated sub crossovers on the market but they have disappeared. If you can find an NHT somewhere, they were great and cheap enough to let you add an AntiMode 8033 for EQ. I also recommend adding the 8033 to the P2100, if you can.