subwoofer to speaker connection

Thanks a lot yogiboy.  The information really helps me to understand the connection better. I have now connected one pre-amp output to my amp, the second pre-amp output to my sub. I like to seek opinion of
1.whether it will be better to use the Hsu Research high pass filter so high music tone can be send to my amp when connecting the sound source from the pre-amp. This will allow my sub to provide the base sound, with assumption that the quality of base is better from the sub than my speaker.
2.sub placement: The manual you recommended stated that " If the A/V receiver (AVR) equalizes the subwoofer channel during auto-set-up, corner placement will often provide the best overall performance". For  stereo set up, my amp will unlikely equalize the sub channel. So would it better to place sub on right corner where most base instruments are played from right in an orchestra (I mostly listen to classical music).

Thank you very much

@moses189
I like when using one sub to place it in the center of the speakers. You can move it around to see which is best for you You should start with changing the adjustments on the low pass filter on the sub until it blends with the mains. If you use a high pass filter you will cut off the low bass on your speakers.You won’t know how it will sound until you try it. BTW, I’ve tried many subs and always preferred my system without them. Give it a go,since you have the sub you have nothing to lose !

I prefer high pass filters as they allow your amp and main speakers work more efficiently while allowing the sub to do what it does best.
I, too, am considering the HSU highpass filter for my HSU subs.
I can't find much info on your amp, but it looks to be a nice one. 
I assume your preamp doesn't have low frequency output, so you are going to need a crossover.
For $100 the HSU is a reasonably priced unit. For my main system I use the Vandersteen M5-HP filters which do the same thing but at a higher fidelity point. 
I will politely disagree with yogiboy regarding the benefit of subs. Every system I have added them was improved. 
Audio Physic has a nice tutorial on placement on its' website, too.

Looking at the Luna manual:
https://www.manualslib.com/manual/1141221/Audio-Physic-Luna.html?page=10#manual
I see you can connect the sub with speaker cables (in parallel with your speakers), and use the subs controls to adjust crossover frequencies.
Though using it this way will mean your main speakers are running full frequency. I am doing this currently in my office, but I prefer using the high pass filter as it lightens some of the work for the amp, but it means the sub and main speakers aren't overlapping frequencies, and thus muddying the sound.
B

I like to thank both yogiboy and gdnrbob for sharing their experiences. I especially like to thank      gdnrbob for taking time in looking up the Audio Physic sub manual. Although my sub is an older version, with cross over settings 32, 46, 61,80,105 Hz , the overall capability is the same as that in the manual, except the new manual is lot more clear in the directions. My sub has both a volume and roomgain function to adjust the instrument sound. After reading the responses, I like to further seek input and clarification for further understanding

1.       gdnrbob mentioned that you “like using the high pass filter…but it means the sub and main speaker aren’t overlapping frequencies, and thus muddying the sound”. If I set 61Hz above to amp to run speaker and 80Hz below for the sub using the Hsu high pass filter, wouldn’t I have 61-80 Hz overlap? Would you suggest such overlap  or just 80Hz (or another frequency such as 61Hz)above to speaker and below to sub? By the way, your experience on improving the sound with connection to sub (like that express by rcprince that I first read in Jan 17 audiogon forum)  is what prompted me to explore this possibility since I have a decent sub not being used. (I did try to sell once last year at Audiogon, but no takers)

2.       Yogiboy you mentioned that you have tried many subs and always preferred your system without them. What kind of subs and speakers have your tried? May be your speaker is an excellent one and thus no need for sub.(The other day I listened to the demo of Magico speakers with Chord equipment in a store and found such fantastic sound from the set up which further prompted me to explore the improvement on my speaker since I have the spare sub) My speaker, Focus Audio, is an old one about 20 years ago that listed about US$4000 and the sub(Audio Physic)  about US$2000.  My pre-amp is Transcendent , and amp is Lectron HJ50.  I am looking to see if I can improve the sound from the speaker by connecting it with sub properly,  so I can  truly test if there an improvement.

By the way, I am very thankful to the Forum and Audiogonians in sharing and advising inexperienced individuals like me. Really appreciate the help. Moses189