$566 is a great price for the SR5008,
provided it's from a genuine authorised dealer. If you think a second sub is somewhere in your future, seriously consider the X4000 at the $900 price, for the superior Audyssey implementation that AcuDefTechGuy mentioned.
Also with an eye to the future, Gene's recent
review of the SVS Ultra bookshelves indicated they are in fact a 4Ω nom. speaker with impedance dips to 4.7Ω @ 55Hz and 4.1Ω @ 200Hz. This
Test Report: SVS Ultra Tower Surround Speaker System Page 3 | Sound & Vision indicates the Ultra Center is also a 4Ω nom. speaker with a minimum impedance dip to 2.9Ω @ 89Hz. This should not be an issue for the SR5008, provided you run a sub up to a 80 - 100Hz crossover higher the better), don't usually listen at VERY LOUD levels and keep the AVR adequately ventilated. If you don't meet all these conditions, a power amp
may be in order down the track. What's normally your maximum master volume level (-XdB) on the Onkyo?