Are you talking an Oade mod 706 or stock 706? I've never used the 706, but just from looking at the specs and (loosely) following the thread about it on here, the stock version doesn't seem like a competitor to the MixPre-6 or F8. Personally I'd take the MixPre-6 or the F8 over the 706.
As to deciding between a MixPre-6 and F8, if it were me I'd just look at what features I want most that are unique to each and decide based on that because they both seem to be great recorders (I only have personal experience with the F8). For example, if I wanted something smaller and lighter, I think the MixPre-6 would be the obvious choice between the two. The F8 does have some unique features that are missing from the MixPre-6, and vice versa. Both seem to be pretty great so you're kind of spoiled for choice.