The stock speakers on the Kuros are actually very good. Quite clear with excellent seperation-- head and shoulders above the sound offered by any other brand. Being tv speakers though, they lack a strong low end. It's pretty obvious that Pioneer intended them to be paired with a good sub, hence the inclusion of a sub out, a feature uncommon on tvs these days I believe.
Up until recently, I've been using a pair of decent pc speakers that have a fairly solid subwoofer. However, the Pioneer's sound bar offers superior clarity, and as I've recently discovered, can sound pretty awesome with some low end support. I'm seriously considering just getting a better sub and calling it a day. This combination should be more than enough for smallish rooms like the one my Kuro is in, and I see no need to invest in a full 5.1-7.1 system until I need more powerful sound to fill a large room.
The 500M doesn't come with an onboard sound system and doesn't have a sub out like the tv models though. You can attach/mount Pioneer stock speakers to them however.