very good speaker suggestions overall, although I'm curious why we're locked into USB only solutions, with such limited input/output complexes and pretty drab upgradability
also, can we please, PLEASE not get into "THIS IS BETTER THAN THAT" type statements, as most of this equipment is going to measure very closely, and the sonic differences are based on subjective preference, not rooted in objective fact
with both of those said, and with your budget in mind, I *really* like the E-MU 1820 or 1616m for you, as you'll have more than, more than enough outputs, and the quality is, well, the quality is very special (here I do get to throw words like "better" around, because are ~$400 interfaces that will measure on-par or better-than the LynxTWO, and in some cases (moreso for the 1820m, which is sadly EOL), on-par or slightly under the Lynx Aurora)
now, you can basically connect any and every accessory you could ever dream of, active monitors, headphones, headphone amplifiers, preamplifiers, receivers, mono/stereo power amplifiers, discrete D/A's, and so on
links:
buy:
E-MU 1820M
E-MU 1616M PCI - PCI Digital Audio System
learn:
E-MU 1820 Digital Audio System
E-MU 1616M
now the meta for that page says 1820M, and 1820M looks to be unavailable elsewhere, I'd much prefer that over the straight 1820, honestly idk what you'll get (probably the 1820, as the 1820m had a daughter card with BNC ports as well, and that isn't shown)
either the 1616m or 1820 would be solid performers, yes you'll have way more features than you'll likely ever use, but the quality is there, and expandability is there as well