AFAIK, the "preamp" is always there, whether mic in or line in.
Just the gain changes. I don't know where the official "zero gain" is, but running line in at a moderate level,
say #15/30, might be a good guess.
Unlike Minidisc, for example, there is only one gain stage, marked "PGA" (programmabable gain) on the block diagram (*). Minidisc has a separate mic gain stage, so basically two PGA stages, one for mic and a second for line. So, putting line in on the MD you *do* avoid an extra gain stage, but not so on the Edirol.
(*) The chip is Burr Brown, "tlv320aic32", if you want to look at the specs in detail. This is a single chip that does everything, mic/line in, ADC, MP3 compression (if applicable), headphone amp, etc. The good news is there are no external parts for Edirol to mess up. The bad news is no "mods" for me to do, lol.
Richard