For example, the iPod's headphone jack produces a 1.0Vrms output, which will only allow the amp to produce 50% of its rated power.
Typically, an XLR connector will carry pro audio levels. Most mixing board outputs provide these. The consumer audio standards, on the other hand, are the RCA connector (usually red and white) or the minijack/miniplug (used for portable audio players). The XLR format provides a higher voltage signal as well as two copies of the signal, together called a balanced line. The extra copy of the signal has the added benefit of being used by the amplifier to eliminate noise that gets into the audio link from lights, computers and other nearby equipment.
So the best way to convert an unbalanced, consumer audio output to the Crown's XLR, balanced pro-audio input is with an active converter box -- essentially, a little preamp placed between the unbalanced source and the amp. A simple XLR-to-RCA adapter cable by itself will not provide any boosted or balanced signal conversion, so this usually won't provide satisfactory results.
Converters come in many varieties, but normally you will only need a simple 2-channel version. These should cost less than $100 and can be found at any pro audio retailer. A Google search of "unbalanced to balanced audio converter" will yield several good choices such as the ART CleanBox or Rolls ProMatch.