first off, I'd like to ask you why you're looking for an LDC in particular. I see you have a few dynamics, and the mid-sized Kels. I suppose that it might make sense to add an LDC as you don't currently have one. But, would that be the best choice for you? I dunno.
You've specified a budget, which is fine. Is this your budget for the next few months? Or, could you wait a while and put a bit more $ into something like a good ribbon mic? Do you need a stereo pair?
Perhaps most important, you say that you do mostly instrumental type stuff, but that is such a wide variety of sty;es it's hard to suggest possibilities for you. What is similar to what you're trying to achieve?
Given your needs as expressed, the mics you've listed can often fit the bill. The Kels are supposed to (I haven't heard them yet) be pretty good on drum overs, acoustic guitars and guitar cabs, among other things. 57s are decent on things that are thwacked or swatted (ie drums) especially if they don't need to capture too much thump. They can, with the proper pre, be good at a lot of other stuff too. They, and the 609, are often heard on guitar cabs on recordings that could afford any mic on the planet.
Not sure about either on horns, but the 57 or the 609 can be decent on percussion (depending). Some love, and some loathe, the 609 on guitar cabs. (I'm more to the loathe side, but can be ok on a cleanish kinda tone).
It sounds to me like the mics you've got, while budget, are pretty ideal for your needs, or at least a real good start. Might there be better mics for a particular situation? Perhaps. But for what you're doing, it sounds like you've got a solid foundation to work from.
However, you don't have a dynamic that captures the lows well with smooth(ish) highs. These can also be good vocal mics (real good vocal mics) as well as gtr cabs, horns, bass cab etc. Mics like a Shure Sm7 or EV RE-20 or Senn 421 or 441 might fit the bill as much as any low to midbudget ldc. Depends. Not often the choice for acustic gtr but, it depends.
Well, small diaphragm condensors might fit the bill too. Don't shut them out either, although they may be less amenable to vocals etc. Depends what you're looking for though.
You don't have a "modern" scooped (low) mids, hyped highs mic. Do you want/ need that? If so, there's a bazillion of 'em. I honestly wouldn't know which to recommend. (this may be exactly what your question was asking?) But, do you think patterns other than cardioid would be useful? If your room isn't too good, that's always a consideration (even if your room is good it should be a consideration). A multi pattern mic can be agreat thing in your arsenal.
How about a ribbon mic? Totally different sound from the 3-5 bills condensers. Depends what you need.
Shoot us a bit more detail what you're looking for. The mics you have now are a good start. If I was to recommend 1 all around mic for everything (if I could name only one) it would probably be the Senn 441. Its a plain jane, though pricey, dynamic. Doesn't sound like one though.
But, if you're only looking to record your voice, as you said, nothing works on everything. Use your ears etc etc blah blah blah. What's lacking (if anything) with the mics you currently use? Is there anything in particular tha's bugging you, or do you just want to expand your palette?
Ok, enough jabbering from me. Hope this helps rather than comes across as a harangue (it's not). You have good mics. There's a bunch of ways you could go though, and all could be valid. Jeez I've typed a buttload and haven't come close to even attempting to answer your question. Uh, I'm gonna go have another beer. Give a bit more detail about what you're trying to achieve, and hopefully you won't have to wade through this kind of tripe again!
Cheers,
Geoff Ruby