Thanks for reply, however, wishing to remain a married man means another amp is out of the question.... As for the speakers and upgrading them, well I was only allowed the ones I have because they're 'gloss white' and compliment the decor...Ok. If you start using it for music, I think you're going to get the best improvement for your money by adding a stereo amp in addition to the 507, rather than upgrading to the 609. That'll do a better job in stereo than the 507 does in 5.1. Probably better results for non-5.1 broadcasts too.
This does, of course, mean that you'll need to have space for two amps and a speaker switch (to share the same front speakers between both amps), so that solution might not be for you.
Choose a second-hand stereo amp carefully enough and you might even have budget left over to upgrade your front speakers too. If you can do this, then you're definitely in a better place than upgrading to the 609.
As for whether the 609 is much of an upgrade for you, I still don't know. If you want to listen to stereo broadcasts in 5.1, Onkyo may well have upgraded the upmixing processing.