Here is my report.
You listen to music the following way: your computer + DAC or ODAC + AMP + headphones or speakers.
Cambridge DAC100 is the "DAC" part. Then you will need an amp(lifier). However, if you listen via headphones, you need headphones amp. DAC100 works with (speakers) amplifier. It has RCA output. It's "too loud", (distorts sound no matter what you do) for ANY headphones. (even 300 Ohms ones).
Then there is "Audioengine D1" DAC which is cheaper (perhaps because it's more specialized) and sounds considerably better via headphones, just because it has hi-fi quality built-in headphone amp as well.
Then there are BLUE headphones that don't need separate DAC and also have their own Amp inbuilt. However, these are "closed" headphones. If you listen to classics or strings exclusively - you won't like them: too much bass, no "air".