Can I use my headphone's with the HD4's?

You can absolutely use your headphones with the HD4 speakers.  The HD4's have a built in headphone amp and a 3.5mm headphone jack located on the front of the powered speaker.  When headphones are connected, the audio coming from the HD4 speakers will be cut out so as to not interfere with the audio coming from the headphones.  We recommend using headphones with impedance levels between 10 ohms to 10,000 ohms.