I've personally used some of these "inline preamp boosters" and have even built a few of my own. While they usually sound good, rarely are they less noisy than just having a good preamp would be. That's why I keep around my banks of mic preamps for my studio - got an old Focusrite Voicemaster channel strip that was very nice for the price of 40 bucks, a Digidesign Pre (8 channel, VERY quiet, nearly 70db gain!), an old Altec mixer I converted to 6 preamps (transformer input, low noise op amps), and a bunch of preamps of my own design, which I managed to squeeze out 70db of gain with less noise than an API or SSL. I've personally never had to actually USE a booster with any of those, though I have with my RME's built in preamps.

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