"If you're using software to emulate a U87 (for example), that software isn't going to be able to take the signal and suddenly make sounds that didn't exist suddenly appear."

Actually, that may be exactly what it does. In a similar fashion, Jimmy Page re-mixed some of the Led Zeppelin recordings and the new digital recording hardware could capture low end frequencies that could not be captured with the original hardware "making sounds that didn't exist" (although they really did, the vintage equipment simply could not reproduce it) suddenly exist. The frequencies were there all the time but outside the dynamic range of the equipment of the day. It's possible the software emulation can do the same if the physical characteristics are the same that the frequencies were just not being reproduced prior to being processed by the software.

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