Proportionality and weakest links in the chain
Assuming that the technical measurements of the cable are as you say, then I'd say the cable - or indeed any cable of similar specification - was technically perfect. If that is so, it beggars belief how one type of technically perfect cable can sound different to another technically perfect cable doesn't it.
Let's put this all into sharp perspective. You say that 2.5m of this cable has a DC resistance of 0.018 ohms. So that's 0.0072 ohms per metre (which strikes me as almost unbelievably low unless the conductor is really fat). However, let's take your figure as correct. You are planning to run 7m so that's 0.0072 ohms x 7 = 0.0504 ohms per cable run from the amp to each speaker. Does that sound a little or a lot?
Ok, let's have a look at this then. Let's use round numbers for convenience. Let's say that the speaker is specified as 5 ohms impedance. Let's divide the 5 ohms by the cable resistance of 0.050 .... that means that the cable resistance is only 1% of the speaker's specified impedance. Can we ignore the cable's resistance as it's such a tiny percentage of the speaker's impedance? I agree - yes we can. It's probably many percent lower than would make any measurable or audible difference. What other resistances are there inside the speaker? Don't forget the crossover coils. They typically have a resistance of about 0.5 ohms (ten times greater than your 7m of speaker cable to the amp!).
My point is that there is a lot of resistance inside the speaker box and none of it is 'good' as it all wastes power; consequently there is absolutely no need to select speaker cables for super-low resistance because the dominant resistance will be in the speaker box not the cable to the speaker by typically a factor of 95:1 (or in your case, 99:1). The extra resistance from the cable will add an utterly undetectable 1% to the speaker's impedance (resistance) so can be completely ignored. No human ear can detect 1% change in level. 10% - maybe.
I always feel that what we forget is that the performance of a reproduction chain such as that from the microphone to your ears is always governed (or limited) by the weakest link in that chain and that the weakest link(s) may not be under your control no matter how much money and effort you invest in improving all the other links. The weakest parts of the chain are those involving any, all and every electro-mechanical components that is, the microphone (and the user has no influence over that) the pick-up/turntable and the LP cutting mechanism (the user has no influence over that either) and of course the loudspeaker. Money spent on the turntable/pick-up and loudspeaker will definitely reward you. Money spent on all the other parts of the chain will offer a much, much lower bang for your buck if any at all.
I see that this is my 999th posting here; I do hope that some of it has been of interest in guiding you to achieving the very best possible sound without wasting a cent. For about thirty years I've been cautioning hi-fi listeners about counterbalancing what they truly, honestly, passionately think that their senses are telling them with a more cautious, disciplined approach. Perhaps now the economic situation will force many to re-evaluate frittering money on gadgets and gismos that they admit to themselves, months later, actually didn't increase the fidelity of their system one jot. Such as - and I pick this from a long, long list of abandoned crackpot ideas - colouring the edge of your CD with a felt tipped pen - once an absolute 'must have' craze. Remember it?
I hope that you've seen some of the videos (in Designer's Notebook) we've started to roll-out about how the design process actually works at Harbeth. You'll see that there is constant attention to the delicate balance between what my ears tells me sounds right and what the test equipment says measures well. It's critically important not to allow your ears to seduce you; something they are very, very capable of doing!
Alan A. Shaw
Harbeth Audio UK