headroom, distortion and clipping, or pres in digital domain

[jislober]

First, could someone give me a concise definition of headroom and what it has to do with signal to noise ratio?
Ok, so I've got a bunch of different mic pre options in my studio. I've got two custom mic pres, a tube one with altec transformers and one that's supposedly a neve clone. I've also been using a Yamaha m508 desk, which actually sounds pretty good to my ears. In terms of tone, the Yamaha fits in kinda between the two custom pres I have. The tube pre is really bright and has a big low end. The neve clone seems really mid rangy, and is really colored, kinda brings everything up front but in the process seems reduce the overall dynamic range of the source. I use the tube when I want detail, the neve clone when I want color, and the Yamaha mixer when I need something in-between. Oh and then there's the shitty built-in pres on my digi001.
Now here's the issue. Before I upgraded the converters on my digi to an old DCS 900 48k, I was never had any problems with clipping. I could run the levels really hot using any of the pres and its was fine. When I switched over the DCS, I suddenly noticed that the tube pres would be begin clipping at way lower levels, while all the other pres worked fine. At first I figured this was just because my the tube pres were too sensitive, especially at the low end, and that somehow the dynamic range was just outside the scope of my a/d converters. But DCS is a top-notch converter, or at least it used to be a few years back, and certainly it's still better than the digi converters. So I can't figure out why the DCS won't take high levels than the digi without distorting.

Oh and I've used a lucid ad9624 and had the same problem.

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Is everything in your chain balanced or unbalanced? or perhaps a mixture?