How to optimize sub integration with 2 channel?

@willemj . thanks for info on hi pass filters and XOs. 
that is my weak point. need to do some research. 
Im interested in the Zu Audio Undertone Sub:

I am interested in this sub: the Zu Audio Undertone subwoofer http://www.zuaudio.com/subs/undertone.
It does say there is a Signal Processing Bypass, but I dont know how it works.

here is a pic of the backside, and connections options: 
From Zu's website:

 "Line-Level Input: yes, RCA (left and right summing)
Sub Line-Level Input: yes, RCA
Speaker-Level Input: yes, 5-way binding posts (L/R summing)
Tunability: yes, very
Signal Processing Bypass: yes, switched for those that want to integrate more elaborate signal processing but desire to take advantage of the excellent amplification of Undertone."

Also: from Zu website, "It is a down-firing subwoofer with active signal processing and built in amplification. It accepts line-level feeds (RCA left and right summing), sub feed (RCA) and speaker-level (five-way binding posts, left and right transformer isolated and summing)."

Here is the best image I could find for the back of the sub, and its connections/options: 
https://static1.squarespace.com/static/51dd8a95e4b0ff2f7c9874ae/59e64781f43b55b96d873030/56d0d5d286d...

@imhififan 
Thx. But let me try and clarify.
The PrimaLuna has a mono out that is used for the sub. I'm not sure if it is the best solution, but I Currently use a single RCA cable to run from the Mono out of the PrimaLuna to the LFE in my current sub (a 10 year old Martin Logan Grotto, 10", 200-250 watts or so). 

Also, the PrimaLuna does have an HT Bypass that only uses the Amp section of the PrimaLuna. Is that what you mean? 
thx

@erik_squires .
thx Eric. I have tried stuffing the ports of my Dali Helicons before.
And tried a variety: 1/4 stuffed, 1/2 stuffed, fully stuffed.
I found that beyond 1/4 stuffed (or so), there seemed to be too much loss of mid frequencies. 
but I'm willing to try again when I get a new sub.

Subwoofers can be as small as they are by using big amplifiers and using them to boost the signal, the lower the frequency. However, there are limits to this if you do not want to have too much distortion, or destroy the driver. Therefore, increasingly subs have complex dsp to do this without overpowering the driver. So maximum spl is monitored, and so is frequency response. Not sure what the Zu does exactly, but my B&W PV1d reduces the low end extension more and more, the louder you play it, to keep the driver within comfortable limits. So there is little point in bypassing this. It does not look like the Zu dsp has anything to do with dsp room equalization of the kind built into e.g the bigger Velodyne subs.
High passing a sub only makes sense in two very specific situations. The first is if you cannot play the system loud enough, either because your amplifier does not have the power, or because the speaker cannot handle more power. If that is the case, highpassing the main speakers and main amp makes their life a bit easier. The second issue is related, if the speaker distorts too much when played loud. Speaker designers can increase low fequency extension, but at the expense of increased distortion. If you do not like the balance that was struck by the speaker’s designer, reducing the burden on the main speakers helps. In short, in normal situations I do not think there is much point in high passing the main speakers, given the disadvantages. And I would certainly not want to spend big money on addressing the shortcomings of an amplifier or speakers.
The Zu looks like a pretty ordinary though perhaps a bit expensive subwoofer. But I have never heard it, nor would I think that a listening test would have helped me much. Subwoofer sound is largely determined by the room they are in, and by the room modes in that room. Listen to them in a demo room, and you have heard the sound of the demo room rather than of the sub. It is for that same reason that I think multiple smaller subs are far preferable, because they smoothen the response, and even so preferably combined with dsp room equalization by a unit like the Antimode 8033. See here for some introduction: http://www.acousticfrontiers.com/20101029using-multiple-subwoofers-to-improve-bass-the-welti-devanti...
For that reason, you may want to consider the Audiokinesis Swarm System, but always in combination with dsp room eq.

Why don't you call up  or email Zu and get their take on it?
From my experience, they are very communicative about their products.
Bob