It’ll somewhat depend on what you want to drive with it.
I enjoyed the A&K SR15, which is usually $699, but there’s one for $549 on headphones.com at the moment. It’s deceptively tiny, as well … which is a good thing in general. There’s a thread on it, here.
The FiiO M11 Pro is decent, as well. Though I’d take the A&K over it, especially if the primary use-case is for IEMs. FiiO tend to have really interesting hardware, that is sometimes (quite often with the prior generations) let down by firmware/software issues, so it’s worth doing some reading there.
Beyond that, again, it depends very much on what you want to drive with it. If it’s IEMs there are lots of less expensive, feature-rich, options. If it’s full-size cans, then herd thins rapidly.