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.