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New Jersey, USA

I'm thinking of buying nanoloop 2.  What gameboy has better sound quality? A GBA or GBA SP?

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Melbourne

I haven't tried nl2 on GBA/SP, only on the game boy micro (which is a bit noisy but seems to be a different kind of noise between all three micros I own).
If you're not planning to sync it to anything, the DS lite or even original DS has the cleanest sound. Unfortunately there's no link port though so you're limited to using it on its own, or syncing by ear.