I recently went that route with a HPF from Merchand Electronics. It's 24db/oct at 80hz. Its a vast improvement in bass control and essentially turns my 3 way speakers into 4 way. Once I heard the result I added a 2nd Sub. I have a small room and chose the SVS SB1000 for their size and the great phone App for setup and tuning.
External crossovers for subs
This might be a stupid question but I'm asking anyway. I have two subs connected via Speakon connectors so they get fed the same signal as my stand mounts. I think I've gotten the subs and mains integrated very well (at least for my tastes). I've read here, on several occasions, that sub integration cannot be done properly without an external crossover to take send only the appropriate frequencies to the mains and to the subs. So, what is the difference between an external crossover filtering out frequencies and the internal crossover of the speaker filtering out low frequencies that the speaker cannot play? Can't an internal speaker crossover be built to filter out frequencies below the threshold of that which is suitable for the speaker design? Isn't that what crossovers do within the speaker by sending high frequency signals to the tweeter and low frequencies to the woofer? Why would it not be able to filter out frequencies beyond the capability of the woofer?