This exact question has been discussed many times before, but yes sometimes they can get a little out of hand and become difficult to get a grasp of which is really better. I have never owned nor listened to either, but FWIR, the PRO 750 is better. The HFI-780 is considered as too bassy and the bass drowns out the mids a bit. When it is modded by ALO, called the ALO-780, it is a very, very good headphone for the money. Most people say it's 90% of the ED 9 which costs a heck of a lot more. The PRO 750 is more detailed and is generally considered as a less 'fun' sounding can and more of a 'critical listening' headphone. I myself have some ALO-780 coming in by the end of the week. They can be had for around $350us shipped worldwide used on here in the FS forum. I would really consider saving some extra cash to fund for them yourself. But if you can't afford it, I would go with the PRO 750 if you don't like overpowering bass. I think the PRO 750's bass will be more than the DT 880 and DT990 you have, probably more than the DT770 but the HFI-780 will be a lot more powerful but at the expense of detail.