From the 503' Manual:
2 channel driven: (All at .08% THD)
North American:
75 W + 75 W (8Ω,20Hz–20kHz, FTC)
European:
100 W + 100 W(6Ω,1kHz,DIN)
Asian, Australian:
130 W + 130 W(6Ω,1kHz,JEITA)
Dynamic Power:
180 W + 180 W (3Ω,Front)
140 W + 140 W (4Ω,Front)
95 W + 95 W (8Ω,Front)
I currently have the HT520 reciever (Comparable to the last years 502 but they put the 520 in a HTIB). The spec's have change quite a bit for these recieves this year.
The 520 is rated at 130 W per channel (Still .08% THD). As you see above they rate the 503 at 75W per channel.
I do not understand the Dynamic Power. It looks like it can push 4Ω speakers. With the 502 audioholics did a review of it and I believe the reviewer had 6Ω fronts at which he expected the 502 to "baulk at" but he was quite pleasantly suprised.
I do understand that this is an entry level reciever and, to answer the poster's question, I woud not use the 503 with 4Ω speakers. I would get a reciever with preouts and and seperate amp. And yes you can use a stereo amp with your home theater. The preouts send the info via rca's to the stereo amp which only amplifies the fronts while everything else still is amplified by the receiver.
If you download the 503 manual the as you see above it seems Onkyo think depending what country you live in you may get more or less power out of your amp. Looks like we should all move to Australia (I do understand it has to do with different ways of measuring the power). I realy wish there was 1 standard way to measure it for all manufacturers to use.
In the manual it also looks like this things more or less depending what country you are in. Look at this from the manual:
Weight
North American: 9.4 kg 20.7 lbs.
European: 10.1 kg 22.3 lbs.
Others: 10.2 kg 22.5 lbs.
To clearify. I have only ever own Onkyo receivers and believe they are great product. I just don't understand this rating double talk.