I think KRK is a good buy.
They keep value, they have focused on speakers only for a long time. (I dont know who owns them these days with background mergers always going on)...
I hooked up an old passive set to a rack power amp, and I couldnt tell them from the actives I have. So passive and amp is an option if you have room and a rack anyway, who cares?
I do believe in getting a 6"-7" driver...with some power try for 100watts, maybe 25-or more on the tweeter in active.
For me and my past tests, I could truly hear a difference in 4", 5" 6" and 8" drivers. MOre power seems to help a lot too.
The triangle setup and some basic room treatement helped and was actually noticeable to the ears (not just a graph).
$400 is a tough price, a lot of great posts on the original M-Audo BX5 or whatever they were, rarely you might find a pare for $180 or $200. Maybe an option but most said they came to life with a sub, and I had a pair for testing and a sub left me agreeing.
Sound on Sound did a shootout years ago, and passive B&W 300 or 301 something, with a separate amp, were rated really good and those can be around that range of $400.
I bought some hi end JBL's in a pawn shop once for $150 pair, and they were passive but the clarity was amazing, though they seemed harsh to my ears, they had more definition than most I have heard.
I might say Behringer but they have more breakdown posts than most, though you might get lucky with a good pair?
just my two cents..... $400 good luck