Here is the model I use that the NX55 replaced..
http://www.yorkville.com/products.asp?cat=31&type=71&id=129

It's official - I have placed an order with a local music shop for two NX55p's. For the price ($650 each) it seems to be the best choice out there for my one-man-band gigging needs. It is supposed to have excellent sound, great bass response for a 12" speaker, good build quality, it can get rather loud, and it is one of the lighter units in its class. For really small jobs (such as those in private homes) I may just use my Motion Sound; at large gigs it will probably find use as a monitor for myself and any singers. Once again, thanks everyone for all the help.

Marc, I think you are going to like the 55's..I think in small rooms you can still use the Yorkville's [1 or a pair]..They sound just as clear at softer volume levels..The one thing I wished my 520's had was a lower bass response[I use a sub woofer because we do dance music]...They answered the bell with the 55's[45 low frequency]..Even the great Mackie 450 doesn't go there....

You did good

Good luck Marc you made a fine choice.....the Yorkies will kick butt for sure!!

Fave Fun

UPDATE:

I received the NX55p's yesterday. I have not yet had the opportunity to try them out at anything more than a very low volume, but here are my initial observations:

1) These are BIG mommas. I don't think they are bigger than typical comparable speakers, but still, they are bigger than I expected. Not that that is a problem, it was just surprising. The boxes were huge and I figured that there must be quite a bit of styrofoam and packing material, but there wasn't all that much - just enough bubble wrap to cover it well.

2) While they are considered relatively lightweight, they are still pretty darn heavy.

3) They seem very well-built and there looks to be good attention to quality by Yorkville.

4) Even at the low levels I was testing it out, this speakers has an amazing bass response - I feel very confident using the two speakers without a subwoofer. I may even need to cut some of the low EQ (around 60-100 hz) depending on the application. Luckily I can do that rather precisely with the PSR-3000's built-in master EQ, not to mention any EQ control on a mixer I will be getting.

That's it for now.