I suppose there's two kinds of 'not hot enough'.
In my experience, tracking with the preamp too low, resulting in a signal that's not hot enough usually means that you're more likely to introduce noise when boosting the signal later.
The reason for that being that the signal level is much closer to the noise floor than it should be.
The other kind is not having the source loud enough, which I've learned can really just make things sound weak and pathetic.
Whether it's half assed vocals or a guitar amp turned down low, turning the software track up never sounds the same as capturing it louder.
I don't know how useful that is to you, but that's my experience.