Connecting Sub to my preamp

My preferred method for subwoofer hookup (and best integration) is to run the sub parallel to the main speakers off the speaker outputs of the amp, or from the speaker inputs themselves to the high level inputs of the sub, as opposed to from a preamp output to the low level inputs.  Allow the main speakers to run full range (avoiding running the subs high pass filters through your main speakers), and only use the sub to augment what the mains do. 

The theme here is to feature the speakers, not the sub.  Set the low pass frequency of the sub at a very low setting (40-50hz if possible), then set the gain very low so that you only hear the sub in very bass heavy passages.  You'll need to experiment with sub placement and the phase setting.  If done well, you'll barely hear the sub, except on passages where there's a lot of bass content, and even then it should be more felt at the lowest frequencies than heard.  YMMV, but this is what's worked best for me.  

I definitely want to follow what you are saying. What is your sub? What are low and high level inputs on the sub? On my current sub I don’t even have speaker inputs.

That definitely narrows your setup options then.  Just use the inputs you have and experiment with placement, and gain settings til you like it. 

I can’t post pics on this forum, but you can certainly google to see the numerous types of input (and output) options on the plate amp on the back of many active subs.

My sub(s) are Definitive Technology PF-1500 and Dayton SUB-1200. Without getting too wordy with details, both are active subs with plate amps built into the backs. Both offer a couple of input options (many do). 1. RCA phono jacks that are low level inputs from a line source like a preamp output. 2. They also have high level speaker inputs that accept the speaker output from an amplifier....they basically reduce that amplified signal down to a lower voltage that’s similar to a line level voltage so that the sub’s amp can then re-amplify the signal and drive the sub, so the main amp only has the responsibility of driving the main speakers. Many subs also have a variable low pass (and high pass) crossover filters, gain knobs, 0 and 180 phase options, etc.