The options for stealth gear are fairly limited, with plenty of choice for low price, almost none for middle price, and a few options at high price. As such, I don't think you can make substantial improvements to the stealth setup for $500-700.
On the other hand, with open recording gear, there tends to be a lot more choice and a lot more middle-priced gear. But before answering your question...
Where is the best place place to put the $$ where my ears will benefit most?
...I think it's important to ask yourself: what sonic characteristics do you like and dislike with your current open-recording gear? If you can point to specific sonic characteristics you like or dislike, you can then evaluate other gear based on those preferences. Otherwise, it's simply an upgrade for upgrade's sake, and you could spend the cash on something else...like attending more live performances, other priorities, etc.
From a generic perspective, you'll achieve the most audible differences by changing your mics, as -- all else being equal, like sound source, location, configuration, etc. -- they have the single biggest impact on the sound you record.