From what I've noticed the receivers for the 770,780 and 790 go like this.
770 = HTR 520
780 = HTR 530
790 = HTR 540
The 520 is equivalent to the TX-SR502, HTR530 = SR503, HTR540 = SR504.
The power readings are different on the HTR systems because they measure them at 8ohm, 1khz while the TX-SR receivers are measured at 8ohm, 20hz-20khz. The TX power is more accurate because it uses the whole audio spectrum while the HTR readings are done to sound like they have more power, 110 watts vs 75. But actually both receivers are the same. The HTR540 and TX-SR504 both weigh 20.7lbs and look exactly alike.
On the less expensive 570, 580 and 590 systems, the receivers are the HTR 320, 330 and 340. The standalone receiver equilavent is the TX-SR302, 303 and 304. BUT THERE IS A DIFFERENCE
with the sub output. The HTR systems use speaker level sub ouputs with the amp built in the receiver, whereas the TX-SR30x receivers actually have a line level sub out.
Saw this on a website for the HTR-330 being sold as a standalone receiver.This receiver is IDENTICAL to the TX-SR303 except the following: 1. Subwoofer output is SPEAKER LEVEL (built in 100w amp) and NOT RCA. If connecting to a powered sub use speaker wire to amp inputs. 2. Amplifier is upgraded from 65w/channel to 100w/channel! 3. The receiver was part of the packaged HTS580
I don't believe the power part though. Its probably not any more powerful than the 303.
I own the 573 and it is XM ready. So are the 540 and 504 receivers. The buttons are just placed differently on the 573 faceplate than the other two and the 573 comes with a better remote with macro buttons. The 573 came out over a year ago whereas the 540 and 504 are recent. The 540 and 504 also have the A/V Sync feature which helps sync up the audio with the picture on flat screens. 573 doesn't have that. I wouldn't be surprised if they phased out the 573, which is probably why I got it for $199 4 months ago.
You can see the manuals for the different Onkyo receivers here.http://onkyousa.com/download/own_man...m?cat=Receiver