I'm not expert here, but I'm in the same boat. Awaiting delivery of a 500m, and decided to go with just bookshelves for now, plugged directly into the monitor (two Polk Monitor 30's). They are not powered speakers, as I've read you need a leveler in order to use powered speakers with this monitor. From what I understand, most soundbars are powered speakers, so you might want to do a little more research.
Hopefully, the Polks will do fine a low volumes until I get a receiver set up. I have an old Sony I can use, but we'll see.