If you are asking whether there is a difference between unbalanced interconnects with RCA plugs or balanced interconnects with XLR plugs, the short answer is No. There are longer answers, but they result in the same No answer. For your purposes (connecting a preamp to an amp), balanced interconnects are irrelevant.
I thought you said earlier you had a Rotel amp and were thinking of getting a Mac preamp. The Denon X4200 has no XLR jacks. Does the Rotel have both RCA and XLR jacks?
Nice looking
. As you have found there is no magic in speaker cables. Making them is not rocket science. No should spend "rocket science" amounts of money for them.
You were getting questionable advice, and I'm glad you questioned it.
Without any doubt, speakers are "where the rubber hits the road". As long as you have enough power to drive them, without causing the amplifier to go into clipping, your electronics are doing the best they can do. And to paraphrase TLS Guy, nearly all preamps available today (including the preamp sections of AVRs) do very well. In fact, you have to work hard to find one that alters the sound – i.e. a preamp that does poorly.