My local hifi store brought up a good point; they said that the only thing between your amp and your speakers are the cable.
Absolutely, but talk is cheap. Don’t believe anything or anybody, just listen yourself and make your own decisions. Has the dealer not done a demonstration for you, comparing what improvements can be had by moving up the speaker cable ladder? Same goes for pre-amps. It is always nice to know what is possible, even if you can’t afford it today.
I think the question you should be asking is what is the weakest link in your system. I don’t know what your NAD pre- section/DAC or WBC speaker cables sound like, but maybe you should be upgrading the DAC first? And even before that: Are you using the manufacturer’s interconnects that came with the equipment? That would be the first place to upgrade- the best speaker cables or preamp in the world won’t make up or re-generate the details lost by poor interconnects or an inferior source upstream.