Dumb Head Phone Question

Sennheiser also makes microphones, lots of 'em, so they are not a dedicated headphone company. That said, I love my Senn 600 phones. Most folks favor the darker, warmer 650 model. I have not heard the 700's. I have a very inexpensive headphone amp (120.00) which pleases me to no end.
You should check out the threads at head-fi.org. That is a "headphone only" site and there's lots of good info there. Don't get overwhelmed.

I own the HD 700s by Sennheiser and like them very much. I also own AKG 7XXs and K701s in the past. They all sound very good using dedicated headphone amps. They aren't terrible using built in amps, like the from Oppo or other gear they just sound a lot better with the headphone amp. It is up to you to decide how much you want to spend but you really don't need to spend a lot to get decent sound.

Great headphones like amplifiers. Might I suggest Schiit Audio as a good place to start.

Also, The Oppo does not have a amplifier inside it to drive headphones. It has a 1/4 inch jack so you can plug a set of headphones into it, but it will not drive a pair of high quality HD700's you need a external headphone amp.

I don't get it. If there's no amp at all, then the jack is useless. Why put it there?