defective pre-amp

Bought a new C47 McIntosh and hooked it up to my Sony UPD800X DVD player.
I have a new set of headphones that are 103db sensitivity. I used a new Pangea Coax cable.
When listening thru the headphone Jack I get lower volume than normal and lots of hiss. I tried 2 DVD player and same thing. Any ideas? bad headphone amp inside or bad DAC? I am next going to try a different cable....

When listening thru the headphone Jack I get lower volume than normal and lots of hiss.

I take it you looked at this, sounds like your still muted, with hiss and low output level.
The C47 Default Setting for using Headphones is to automatically mute all the Output Connectors when the Headphone Cable Plug is inserted into the C47 Front Panel HEADPHONES Jack. There are three available settings: Mute All Outputs Mute Main Output Mute No Outputs 6. Rotate the INPUT Control until “SETUP: HEADPHONES, Mute All Outputs ” appears on the Information Display. Refer to figure 26. 7. Rotate the VOLUME (ADJUST) Control to change the current HEADPHONES setting from “Mute All Outputs” to either “Mute Main Output” or “Mute No Outputs”. Refer to figures 27 and 28.
Cheers George
Are you listening to CDs, DVDs or SACDs? Might want to go into the setup menu of the DVD player and try changing the digital audio output to PCM. Worth a shot.....
Thanx guys!  I tried the both adjustments and nothing worked! Baffle now! Any other adjustments. I tried 2 different cables now, nothing! I tried headphones on my computer and TV, they worked great!
I doubt its a bad dac. If its an open box item its best to reset to factory settings and try again. And have you tried another source or output through speakers?
No got it for headphones and to use with my DVD player to play CD's. Only play CD's presently. Might add turntable later and maybe an amp.
Only other thing I can think of is to check the Sony's settings to see if you are set to send audio through coaxial and not just HDMI.
McIntosh's headphone amp may be more suitable for your needs. It can play small bookshelf speakers as well should you need to expand its use later.