Vocal to the left, instrument to the right

That does not sound right.  I would assume it is the recording.  Try some other selections.

Agreed, that sounds like a choice by the artist or mixer. I don't see any reason your music should all sound like that, unless you have partial hearing loss in your left ear that would emphasize mids... (lost bass, for instance)

It could also be the headphones, if you did not try any other ones. Channel balance variations of up to a few dB throughout the audible frequency range are not uncommon, especially with cheaper models.

Track down a mono recording or test tone to determine whether it is actually channel imbalance somewhere in your chain.  

i have hearing imbalance i think, as i always hear a little more from the right side, i get it with most headphones, some more than others.

Well, the first step is to diagnose where the problem actually lies... if you are using the usual red/white RCA connectors somewhere in your chain, just swap them temporarily. Then listen: are the voices/music switched as well? If no, or for instance the voice that used to be full left is now almost centered, but not full right, it may indeed be a hearing imbalance. If yes, then it's most likely not your ears but the chain and/or recording.

Actually, most likely the recording. You can find test sounds/tracks that play full left/right. If those sound correct, then it's the recording.

If not, one more test of your equipment chain is to try to play back a mono recording (be sure it is!) like somebody else suggested. It should be centered. If it isn't (properly centered, or full left/right), try removing each part of your chain. Example: plug the headphones straight into any other source (laptop, CD player headphone out, MP3 player etc.) If they sound fine, try your amp using an analogue source, leaving out any DAC you are using and so forth.

The important thing is to do check one thing at a time, it will be more time-consuming to narrow things down if you change several things at once, or just use a variety of music tracks which may all have unique left/right mixes that are difficult to clearly nail down.

As far as hearing imbalance goes, have them checked. It may be as easy as having them cleaned (carefully!!) (also:http://www.head-fi.org/search.php?search=ear+clean)

I believe that it is a source/mastering issue. I have heard this at different times depending on the material.

Just my .02, but it's most likely the recording. Great example is Beatles albums. Almost always have more going on in the left channel than the right. An easy fix could be getting an audio player with crossfeed.