I think the higher you go up the hifi chain the more a decent speaker can make. But of course this is down to the listener and what you are looking for, small nuances, a tad more bass, perhaps more relevant for someone who has spent a grand or more...
I've just spent the best part of £400 on new mains leads that are too upscale for what I have now, and whilst I've been pleased with the changes, very pleased in fact, I bought them to last me through an upgrade or two, which will be for very expensive kit if and when I can afford it. 'er indoors doesn't know about the cables.
But in answer to your question, yes, but on a £300 system it may be overkill to spend more than £2 per metre for some copper cable.
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