It really depends on how much you want to spend. All the gear reference in your original post is all decent gear, pro-sumer stuff, and you could already achieve good sounds using that gear alone. (notice good, not great)
That little "something extra" gets expensive...fast.
API 512c, directly into your 003 Converter would do nicely. They're great preamps on almost anything. They have a very fast attack and have a certain high-end quality that sounds great. A stereo pair in a lunchbox will run you about $1400-1800, depending on whether you want to buy new or used.
Lead vocal is something different....Great River MP-500 NV would fit nicely into your new lunchbox. It has a nice warm "Neve-ey" sound that compliments lead vocals very well. They do particularly well on acoustic guitars too. You're looking at $600-800 on those.
All that to say, if you want to start getting top notch sounds, I'd start with technique, then move on to better gear. Starting a collection of modules for an API lunchbox is fine, but know how to use them too.