I seem to have found a solution for my problem. I demoed Chord Anthem Reference as an interconnect cable and a 3m of Chord Epic Twin for the loudspeakers. The improvements were really big - more openness, detail, bass extension and more bass weight. I really liked how it sounded. The bass was considerably better with these cables - Naim seem to really shine with the right cables. Now that I've bought the interconnect which I really enjoy I'm looking for new loudspeaker cables. Although I really liked the Epic Twin I wonder if Chord Signature (the old plain version) would be an improvement over the Epic. Unfortunately, as far as I know the old Signature is not produced anymore and I can't demoed it from a local dealer. I found a good offer online and that's why it I started considering it as an option. It has a bigger gauge over the Epic Twin - 10 vs 12 which I assume can bring even more bass, however the Signature is not silver coated as opposed to the Epic Twin. Have someone of you experience with the two cables? Would the Signature be an overall improvement over the Epic Twin and would the lack of silver coating can have some impact on the details and the highs extension?
If you didn't try you wouldn't know. Apparently it's not the Nait 5SI that needs some time to break in but the speaker cable that's the culprit to the less than stellar performance you were experiencing.
Although the Chord Signature speaker cable may not sound as good as the Epic Twin, it's a bit of an overkill for the Nait 5SI. There are other cables that may sound more open, detailed or transparent than the Epic Twin but some of the bass punch or extension may be lost with the added openness, and they often come with a higher price tag. The Epic Twin is actually a rather good cable as (in my system) it offers slightly more refinement and detail than the NAC A5 without losing much of the boogie factor.
My advice is to enjoy the music with the Nait 5SI and Epic Twin as is.