Matthew, that really sucks, especially when you guys are taking the care to get that mic close to the actors. When you dump the footage and sound into your editing program, try using a Low Pass Filter on the high range of the hiss. Obviously you don't want to muffle the dialogue, so start at the highest ranges, above 5Khz. Since you got close mic'd audio, you should be able to save it. Also, put in a well recorded ambient background track. You can use that to keep ambient noise constant, while you ride the levels on your dialogue.

For future shooting, do a sound test and play the tape back - perhaps on another camera or capture part of it. I'm surprised that the headphones didn't reveal the noise. Doesn't the XL-1 use small input plugs? When you use XLR, you need an adapter, right? My associate producer had that camera, but we used it a long time ago, so I don't remember. If so, it could be your adapter.

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Yeh it could be the adapter i suppose. I think i can clean most of the hiss with Audacity's noise removal on the lowest setting so to avoid digital artefacts. It seems to work better than Sounforges even though its free!

Im gonna lay down an ambient track and there is also music playing in the background so hopefully it wont be a problem. It just pisses you off when your using good gear and you still get these problems!

One option you could try which does take a lot of work is to do a fast fade up from silence (i.e. zero volume level) at the start of each word of dialog and then a fast fade down to silence at the end of each word; we had this problem on a movie I worked on and eventually that was the only way we could solve it. That did leave some hiss on the dialog, but once we put a background track over the whole scene the hiss was inaudible; unless it's very loud it's a bigger problem between words than during words.