I was in a similar situation; good amp and speakers, mediocre pre, Bluesound Vault (which uses the Node 2i and adds a CD ripping ability). I added a used Benchmark 1 for $300 (at the time), and the difference was immediately noticeable; a big improvement. I'm told that the later Benchmarks are much better, and have been hoping to upgrade to a $1K to $2K DAC for over a year; the Gustard X26 mentioned above by @yyzsantabarbara is currently at the top of my list, as is the Brooklyn DAC+, but there are SO many possible choices. When I do pull the plug, I'll be surprised if the improvement is as noticeable as it was from the addition of my Benchmark 1; I'd strongly encourage you to get a better DAC than the one you have in your Bluesound 2i; Your range of $500 to $700 should work, expecially if you buy used. Or, if you're willing to spend more, get a pre with a good DAC in it; either way I'm sure you'll enjoy the difference!
