If you're prepared to buy used and from eBay, and if
audio quality is a priority, then you might want to keep an eye open for Stax units. I do occasionally see sets go at levels within your price range, though not having inspected them, I have no idea what condition they are really in.
But make no mistake, they are
very good units. Put it this way, current models, new, are way outside you price range for the low end of the range, and the reference models are well north of three grand.
So you'd be looking at a fairly old set. Which might put you off. But if not, I've got a couple of sets of Sennheisers (580s and 600s) and the Stax units I also have (and have had for ages) are (
IMHO of course) just in a different league.
I perhaps also ought to clarify what I mean by "audio quality". These are not bass monsters. Their base is, in my view, exceptionally good, but just not heavy. The real signature of electrostatics is that they're beautifully clear, detailed, musical, open, light and
fast. That's why I think the bass is so good. Bass, like drums, is fast and taut. It's as close to the sound of an actual drum as I've heard, and it has that taut punch of the real thing.
And by "light", I mean they are easy to listen to. You can get immersed, for long periods, and not realise it.
That's the good news. Now the bad.
As Stax's are either going to be expensive or fairly elderly, there are some problems. First, you'll be getting not just headphones (earspeakers, as Stax would have it) but an "amp" too. I'll refer to it as an amp, but strictly, it isn't. It's a step-up transformer (as electrostatics operate at high voltage) and a bias. But it looks like a headphone amp. So, you connect that to the speaker outputs (not a headphone jack) on your amp, and the actual phones to the transformer. Some transformers have been known to develop problems.
Secondly, the very thin film "diaphragm" that actually moves to provide the sound can, on the older units, develop problems. If so, it can be refurbished (replaced) but it's a game to do it. And the older the phones, the more likely the mylar is to have issues. Having said that, I bought mine in the 70s and no problems .... so far.
So if you go this route
buy carefully. I would want to hear them in operation before parting with money, were it me. And while they do come up on eBay, they aren't that common, so you may have to hunt around for a while.
Last point on audio quality. It's very subjective. I love the sound, largely because I find it so detailed I hear more of the music and less of the headphones. My Sennheisers are good. Very good. And, superficially, on first listening, might seem better. But give the Stax's a good listen and after a while, you should realise what I'm talking about. But your mileage may vary.
And a final point on Stax's, too. Just as many audio items need to 'loosen' up. or warm up, so give the Stax's a while to "charge" if/when you audition them, especially if they've been unused for a while.