I'll start with a word on warning. In my experience the headphone out of an Onkyo A/V receiver is very weak. It probably won't adequately power an HD 650 and certainly won't drive most if not not all planars. It's not like the 70's and 80's when receiver headphone jacks could power about anything you plugged in--and included 600 Ohm dynamic headphones.
If you want a quality and affordable portable amp that will drive almost anything except planars, the HeadRoom Total Airhead is still on sale for half price ($49.00):
You should plan on getting a desk top amp at minimum and perhaps an amp/dac in either one piece (purchase) or two. I'm old school and still do my serious music listening on CD & vinyl. So for me, the amp alone is enough. My guess is that you will end up with both (perhaps in a combo unit). Good desktop equipment that will push planars starts at about $225 to $250 a pop. Some (among many) examples are the Schiit Asgard amp & Bifrost dac, Matrix M Stage and Audio-gd NFB 12.1 (amp + dac). What you select will greatly depend on how it pairs with the cans you ultimately buy.
With regard to cans--the HD 650 (and older more neutral) HD 600 are classics and still hold up very well. Above them the best buy new is the HE-500.
Thanks kindly for the advice, it sounds like I'm better off to save my $ for now and research a little harder. $250 isn't bad at all for a good amp! I'd rather be able to drive some good planars
This site is great!