You can "easily" put one or two subwoofers in a system with an analog preamp if the subwoofer has its on crossover. I just added two Velodyne Optimum 8's into my 2ch system and have gotten excellent results. I just wanted a slight "beef up" of the bottom end in my system. The subs have speaker inputs and line level inputs and outputs handled by the sub's crossover. I tried the speaker input setup first and after dinking with the sub crossover and sub volume I've got the subs merged seamlessly into the sound stage. I'm so pleased that at this point I have no interest in trying the line level route: preamp -> sub -> amp -> speaker, even after buying new interconnects to facilitate the setup. And, obviously, using the speaker input on the sub still allows full range to my speakers vs the sub crossover filtering low end to the speakers with the line level approach. I forgot to add that these subs also have an automatic "room correction or calibration" feature and come with a microphone.
Ich bin hier und du bist mein Sofa