Unless you need more channels, or are going to upgrade to a pretty serious level, I'd suggest just getting some good mics to use with the zoom. You're going to need to spend quite a bit of money to make even a small audible improvement over the zoom. Better mics with the ability to have different mic placement will get you pretty far. Don't forget room acoustics. Together, you get a much bigger improvement than moving to a mid- quality preamp.
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