Originally Posted by PentHassyKon 
I currently have a Pentax A 70-210, which is constant f4. However, based on the results I've gotten I'm wishing for a longer reach or faster (larger min aperture, 2.8 or less).
Longer but not faster is easy - as you say, the 55-300 handles that, the K300/4 even better. Faster but not longer is easy too, although not particularly cheap - DA*200/2.8, Sigma or Tamron 70-200/2.8, etc. Longer *and* faster (eg, a 300/2.8) gets you into some major bucks, not to mention some major size and weight (nothing you'd want to handhold). No way would I go there, and I'm pretty serious about concert photography. If I'm at a concert where I'd need a 300/2.8 to get a usable shot, that's a concert where it just isn't worth taking the camera. Indeed, it's pretty unlikely in many venues that they'd let you in the door with 300/2.8 without a press pass, and if you've got a press pass, you can be shooting from close enough that you don't need a 300 is the first place.
Frankly, for outdoor concerts or other settings where you need 200mm or more, I think you've got a great lens for that already. Sure, a 300/4 sounds nice, but 99% of the time, that's too long, so you'd want the 70-210 too. And it doesn't take *that* much cropping to get 210mm to look like 300mm.
On the other hand, assuming you are at all serious about this, getting something faster in the range you already have absolutely makes sense. The way I figure it is, if you're serious enough to be considering spending a few hundred dollars on a lens for concerts, you're serious enough to make the effort to shoot from as close as possible. It's pretty rare that I need anything longer than 135mm; I'd consider the DA*50-135/2.8 more or less perfect if I wasn't more attracted to small primes myself. But most of my shooting is in clubs, not auditoriums or outdoors. If I was mostly shooting in locations where I truly needed something longer than 135, I'd be looking at the 70-200/2.8's. But either of those zooms would push your budget.
If I were you, first thing I'd be getting is an old manual 135/2.8; that would supplement what you have (the 70-210 and 55) nicely and cover a lot of ground right there, and leave plenty of money in the budget to apply toward something else later.