Right. A lot of subs do have adjustable low pass crossovers filters. They just don't have a high pass filter as well. And you tend to see more of those high level input and output pairings on budget subs, which don't have high pass filter capability. However, I think the SVS SB12-NSD has an RCA input/output option that uses an 80hz high pass filter on the RCA output. It's smaller, too, than the PB12. Check the manual or call SVS. The other problem you'll have is that other than SVS, Rythmik, Power Sound Audio, Outlaw Audio, and HSU, then most of the subs available are really not good price/performance values. For example, to get an equivalent performer to the SVS PB12-NSD from a traditional speaker vendor, you'll likely end up having to pay $1200 to $1500 MSRP. Unless you get them on sale, it's just not worth it.
If you want multiple crossover options, other than an Outlaw Audio RR2150, an AVR is the way to go. Now instead, you could get a mini-DSP
and run your source to it, and then setup high pass and low pass filters for the RCA outputs. If you bought an integrated amp with pre-out/amp in, you could run the pre-out to the mini-dsp. Nice thing about it is that it has a built in parametric EQ if you ever get serious enough to get a measurement mic and work with REW or some other measurement software.
It would be easier to get a Denon receiver with Audyssey MultEQ (or better version) since the room correction software will EQ both the speakers and the sub automatically. I think you'd be surprised how good one of those can sound, especially given the benefits of the bass management and Audyssey.