An A/D & D/A Converter isn't what you want. You'll need a device that converts unbalanced consumer levels to balanced pro levels, colloquially called a "match box", or "bump box". They run about $60 and up and handle stereo pairs in both directions, consumer to pro and pro to consumer. You only need consumer to pro, so half the box will go unused. The less expensive ones don't do the job quite as well as the expensive ones, but might be fine.
But a couple of questions: why the QSC Pro amps? Do you already own them? If not, there are less expensive, better matched amps for this purpose. The QSCs are great amps, and there's a chance you can connect them directly to your Marantz pre outs, crank the amp gains wide open, and you'll be fine. There's a possible level match issue, but not always. Can't find the Marantz specs to verify just now.
All QSC Pro amps have cooling fans that are fairly noisy for home theaters. In fact, most pro amps of all brands have them.
Just to be clear, there's nothing to be gained by the use of balanced lines other than noise immunity. If your cables are short-ish, that's not a problem.
