Save yourself about $1,300 and build some like the ones I have. Search on this site for "DIY power cord".
Since the Dell uses a big, noisy switch-mode PS, I don't think there's much of a point in spending good money on a power cord for it - use whatever came in the box. As for the Mackie powered speakers the power cord should be the last of your concerns - having repaired several of them over the years, my best advice would be to replace/modify the stock power supply entirely; there's bigger problems lying in wait inside that seemingly innocuous enclosure. Good luck.