Try them out man, or can't you open them if you plan to return them? For gaming and/or music with a heavy beat some people prefer the HA-RX700 even tho the HA-RX900 has a wider soundstage, I've read some people complain it's a lil' too boomy and the bass isn't as controlled... Guess it depends what you're listening to. I only own the RX700 so I'm not much help in that regard. I imagine the RX700 and HADX1 would complement themselves the best tho, dunno. Why exactly are ya keeping two of 'em?
P.S. There's a huge thread about the RX700/900 both here and on HeadFi.org, might help you make your decision, look for 'em (30+ pages)... Best bet is just to listen to them yourself tho. You might find the RX900 works best for positioning sound in games for instance and that when you want a better sound signature you can just switch to the HADX1, I dunno. I'm quite happy with my RX700, probably selling the Senn HD-555.
Ahh, I thought you were returning 'em, not selling it forward, my mistake. Sorry I can't be more help, I'm not familiar with the HA-DX1 or the DT-770/880.