KEF LS50 Wireless integration with REL sub

I bought the KEF LS50 wireless back in December and was blown away by the amazing performance of these speakers at their respective price point. Although, I was more than satisfied with the lower frequencies without a sub (the base output and quality was on par with my full range B&W CM10s, believe it or not), when a local seller put out an ad for a barely used REL T9i at a very attractive price, and in matching white, I just could not resist. Integrating the sub with the speakers took about an hour or so. Given the position of the speakers and the general layout, WAF etc, I had very little flexibility in placing the sub. Anyways, to make a long story short, the results were impressive. The sub has blended and disappeared very well. The biggest benefit is that I can listen at a lower volume and still get a very satisfying experience. However, I feel that the imaging has suffered a tad. Without the sub, the imaging was laser-focused. It does become less precise when I turn the sub on.

Now I'm just wondering if it is possible to improve the integration even further. The KEF application provides several settings to tune the sub per your taste. This includes: High-pass mode (I'm not sure what it is supposed to do so right now I have turned it off), Sub-out low-pass frequency (I feel the best setting is 75 hz), Sub gain (I have it at -4.0 dB).

The REL T9i also provides cross over and volume control knobs on the back. The problem is that given that both the KEF app and REL provides a way to tune the sound, what is the appropriate mix of settings? 

If anyone has tried the KEF LS50 wireless (not the passive version) with a REL sub, I would really appreciate it if you can part with some of that wisdom. 
On the Kefs I would have Sub gain set at 0 or nominal, (dont use the controls on the KEF app to adjust sub, keep any filters off let the KEFS do what they do) then add the REL (it should be placed facing the center of the room in the corner to the left or right of the main speakers) follow instructions for adding sub in the REL manual. The REL blends with what the KEFS were already doing so dont change the KEF settings, once you get the frequency and volume dialed in on the REL you never touch it agian, just play the system adjusting volume on main speakers. Its key to dial in sub so that it is in corner (not all the way) of room and the air around it seems to be energized!  The REL does have a voice coil and having it facing properly (towards your listening position) helps integrate with mains. Bass is not just felt its heard also!

Matt M 
shouldn't high-pass mode be turned on to avoid the KEFS from duplicating the low frequencies?
Congratulations. The purpose of a high pass filter is to cut the low frequencies being reproduced by the main speakers. Since the low frequencies pose the biggest demand on both speakers and power amplifier, you make life rather easier for them if you divert that task to a dedicated low frequency unit. However, it also makes integration more difficult because you have to deal with two filters rather than just one. So unless you want the maximum output from the speaker combination, there is a lot to be said for allowing the main speakers to play full range, and only use the subwoofer to fill in below them.
Without having tried your combination I would have thought that 75 Hz is far too high for the low pass filter of the subwoofer in this combination (assuming the main speakers are playing full range). My hunch is that some 40 Hz is a much better ballpark.
My second piece of advice is to only use a subwoofer with an Antimode 8033 or similar system of room equalization. Room modes play havoc with the in-room sound with huge peaks and dips. And their upper harmonics also mess up response at higher frequencies, particularly in smaller rooms. Room eq significantly reduces such room modes. And if you really want to get the best low frequency sound, use a second sub (in mono, and still with room eq). See here:
I really appreciate the advice.

@mattmiller  I apologize for not able to fully comprehend your recommendations. Are you suggesting that I turn off the KEF subwoofer settings altogether and only tune the REL? The instruction manual for the REL describes how to connect but it has no mention of using a third party software to fine tune the settings.

@willemj So based on your advice, I set the low pass filter to 40 hz, and I really wasn't able to hear any base. I then moved up to 50 hz and the base was definitely tighter and the imaging improved. Although, I still have the high pass filter disabled. If I turn it on, not sure what would be the recommended value. Should it be 50?
And I wish I was able to put another sub in the mix but given the current layout of the room, it is not possible. I was barely able to squeeze the speakers and one sub.
As for Antimode8033, I probably need to do some research. I'm not sure how to integrate it with the sub and the speakers, given that there is no separate preamp. I'm assuming it sits between the sub and the speaker (RCA jack?) in the chain (?) Definitely sounds promising.
Maybe you should have increased the level of the subwoofer when you lowered the low pass frequency (that is what REL usally recommends, and I tend to agree). But it is just a thought.  If you really want to know/see, get a calibrated measurement microphone like the UMIK-1 and do a measurement in REW (but not for the fainthearted in computerland). Your original 75 Hz was certainly too high without high passing (i.e. cutting the low frequencies) of the main speakers.
As for the Antimode. If you do not have a preamplifier output, you can connect the sub at speaker level and then use an attenuation cable (that is what I do). Look carefully at the connection examples at the DSpeaker website. The cheaper Antimode 8033 Cinema model is probably all you need.
@willemj  Thank you for the valuable advice. I do have one more question ... since both the sub and the kef app has a crossover setting, do you recommend an optimum setting? Should I set the sub's crossover to the lowest and use the app, or vice versa? Thanks.