This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
Going by the rule that "external is better than internal," the
external preamps are likely to sound better. Besides the issue of
electrical shielding and interaction, it is simply the case that a
designer who is spending *all* his time on a project designing only a
preamp is likely to do a better job of it than a tape machine design
team that has to worry how they're going to fit the preamp into the
box and still have enough room for the rest of the tape machine. [Gabe]
 
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