To me outboard amps are like gas in my jeep. When I use the lowest grade (87) it runs good, I get where I need to go and all is fine, but when I use higher octane (89-91) I get a more peaceful running engine, the ride has a tad more power and all is great.
Same principle. I have the Onkyo 805 (which is almost the same as your 905 but less video processing and one HMDI) and ran my speakers just fine, but when I added an outboard amp, the speakers opened up, seemed to run more smoothly. Granted the amps only improved the sound slightly (not night and day mind you) but id did improve and I can play my system far louder than without the amps.
Enjoy and nice amp.