The converters are the same in the 100 and 700 - I was lead to believe by Cakewalk. The preamps are "based upon" the 700's, which means cheaper components I believe.
To my mind, there is hardly any difference between converters, until you get into the boutique league. The ADDA chips are almost all the same in similarly priced units and only made by a couple of companies anyway. So you won't find much to choose from there. However, the analog stages can be different, and you won't find transformers etc. on most converters like you do Burl etc. (of course, you are talking about a minimum of $6000 for the burl). But any converters built in the last 5 years so are more than respectable.
Preamps are much the same. The Roland preamps are good. But there is only so much sound you can squeeze from an IC unit and the differences are small - smaller than some imagine when they hear a night and day difference.. Once you get into discrete design and transformers (there is that again) you start hearing real differences. If you are happy w/ your mics this is where I'd look. But only if you have money for a good one. And fortunately we are in the age of dropping prices for preamps. The Gap, the ISA One, Warm Audio's warm preamp and their new Tone Beast all are about as good as it gets. You can spend a thousand dollars more and find less difference between any of them and a typical interface preamp. I particularly like the Tone Beast (tho it is $600 not $400) since you can change op-amps and switch transformers. A solid preamp (based on the Warm) but w/ options. And a gain and output, so you can distort it like the Gap. A good ;-) review is out in Tape Op.
It sounds like you just don't have GAS (Gear Aquisiton Syndrome), but have maybe reached the point where you hear some subtle things between your work and stuff commercially done, and at some point you have to step up the hardware. Or room. The order of importance is closer the source, so it is room, mic, preamp (and comp and EQ), converter. That is a good rule to follow. If the room is decent and you are happy w/ your mics, I'd suggest a transformered preamp into your 100 to get to that next level of sound.
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