You can try the new Schiit Lokius. Probably the only thing around that range. Nice thing, it accepts balanced input (but it's not balanced internally).
On the bad thing, well we don't know as the product is too new.
Yeah, SW would be least expensive and most flexible...
If you want an old-school analog, then the dbx1231 is probably as close to $300 (I think it is a little over $400) for a reasonable 31-band stereo graphic equalizer that I can think of off-hand. If you just have a spot or two to fix, a parametric equalizer might be a better choice.
If you are somewhat technically savvy, a little ~$100 miniDSP will let you build all sorts of filters, though I've heard it can be noisy in some systems.