Alright, currently in my S70 I have a double DIN I installed:
It works fine and dandy and it was super easy to fit it. Just shaved off the guide rails in the center console and voila.
However, I may or may not have just sourced a really rare OEM factory trim kit that I'll be swapping in 
. It still has the guide rails and it doesn't feel right shaving the rails off and altering it as it's a rare OEM piece. Because of this I'm considering swapping back to a factory head unit when I swap the center console.
This does bring me to a couple roadblocks but I think I have them solved... First is integrating the sub I'm currently running (Diamond Audio Sub/MB Quart mono amp). I know how to make that work fine with an adapter to run from the DIN amp out on the back of a factory head unit. 2nd is the iPod I currently have integrated in my left vent that plugs into the USB input on the double DIN I'm running now that charges it and also serves as the audio in. I can solve that easy with a factory head unit with a GROM.
Now, here's what I don't know as I just don't have much info on factory head units since I've always just swapped them out. What's my best option for a factory headunit that'll be a direct plug in? When I did the double DIN install I used the Crutchfield harness adapters so my factory harness is completely intact. Will any 850/70 head unit just plug into my S70's harness and be compatible?
I know the SC900 and 901 are popular upgrades but I also know they require an external amp. Do both of those units have 2 DIN amp outputs on the back of them? So I could run DIN to RCA adapter off the back for my sub amp and also still have a DIN amp output for the factory amp that goes with the SC900 or 901? Or does the factory amp for those not integrate via a DIN connection?