Best "laid-back" speaker cable & interconnects for all Naim system?

Here are several suggestions;
I know that Vandersteen recommends biwiring their speakers, so perhaps that would provide a more even frequency response.
 I don't know what your cable budget is, but Purist Audio cables are laid back with a smooth top end.
Also, are you using any room treatments? Absorbion and diffusion are necessary to remove any standing waves and smooth out the highs.

Last item; try posting this question on the Naim owners forum... 
http://forums.naimaudio.com/forum/hi-fi-corner

At one time NAIT's were unstable using other brands of speaker wire. Check with NAIM first!

" At one time NAIT's were unstable using other brands of speaker wire. Check with NAIM first! "

He is using Naim cables. On the same amp the OP has I tried: AQ, Kimber, Tara, Synergistic, Harmonic and Cardas speaker cables. Its also interesting to note that when I tried AQ Cheetah and Panther (identical cables except one is silver and the other is copper), I couldn't tell the difference. It was the only time I couldn't hear a difference between copper and silver.

I didn't think it was relevant, but since you brought comparability, a while back I went to a Naim dealer to audition some new gear, and brought some of my components and a bunch of AQ cables because I didn't know what the dealer would have, and it was a long drive. The store owner asked if he could try some of my AQ cables, and of course I said OK. It was Saturday, and after the owner played around with my cables, he said he was picking up the line first thing Monday morning.

So given all that, I doubt there will be any compatibility issues. But I also stand by my original statement in that new cables won't be enough to fix the problem. 

5i owner's manual states naim cables sound best, but you can use a wide range of cables without causing damage to the amp. 

I have a Naim 5i mkII - and the best commercially available cables I have reviewed comes from KLE Innovations (KLEI).

The gZero3 IC is very good starting point for the price, but what I consider their "best value" is the gZero20 IC (which I settled on) - superb detailing, bass, image and clarity

I currently have the gZero2 speaker cables on my AV system connected to a Blusound Powernode 2 and they sound superb - with a sub
http://image99.net/blog/files/category-klei-gzero2-sc.html

On my audio system (Naim) I use the gZero6 speaker cables because they provide much more depth and bass control than the gZero2’s did
http://image99.net/blog/files/category-klei-gzero6-sc.html

My speakers are Gershman Sonogram’s - a full range floor standing unit

The KLEI TOTL models are expensive and as always, you get diminishing returns, but they also offer improvements over the gZero20 IC’s and gZero6 SC’s - it is just more difficult to discern improvements and justify the additional expense - i.e. for me

You can read my KLEI reviews here (see menu on the left side of the screen)...
http://image99.net/blog/index.html

The reviews are also posted on the KLEI site along with reviews from other people.
https://kleinnovations.com/category/reviews/

Please Note: I am NOT a professional reviewer and I completed that many reviews simply because as I received each subsequent product I could hear improvements right off the bat - my curiosity got the better of me :-)

But as you will see - other reviewers also seem to agree with my findings.

I have tried several different cables from a few brands and the KLEI cables are the best I’ve tried, not only with my Naim but also on a much more expensive Arye system.

Ayre amps also has an issue with high capacitance speaker cables, but the KLE products provided no hint of those issues - just clean, dynamic sound.

The only negative thing about the KLEI products
- they require a long burn-in period to sound their best - from new, upwards of 400 hours.
- they also require about 20 hours re-seating time, so even just moving a cable between components requires time for them to be at their best.

However, their best is pretty amazing!

KLEI also offers power cables, which I have not tried - Yet :-)

But based on their other products - they are probably very good also.

BTW - good power cables make a huge difference to the SQ on Naim amps - their stock cables are better than most, but still leave a lot to be desired

Regards, Steve