Hi M_Glenn_M
In the example you've given it just sounds to my ears like there's just too much of a low cut going on. It almost sounds like there is a fairly steep low cut at about 900hz. Did you check the eq on the vox track for a low cut that got engaged somewhere?
 
Dean

Drop the digital effects going in.  No sense in using them since you can add those after recording - they are post conversion.
 
@

I do have the lows cut below 200 thanks.
I've now cut a few db at 1000 and rolled off under 100
I do think there may be a bit of combing due to vocal doubling.
Thanks guys, you have given me lots of ideas to try

It definitely improved when I cut one vocal track.
I was trying to do the HAAS thing but it made it hollow.

To solve the issue, go back to square one.
 
Mute all the vocal tracks but the main one. Bypass or delete all the FX. Set the EQ to flat.
 
Most people get into trouble when they try to do "fancy things" with doubling, EQ and FX without a firm understanding of the basics of sound.
 
To my ears, the sample you posted was lacking the low end and had too much high end. Try singing much closer to the mic following the advice in the second sentence above.  You can then listen and evaluate the results and use the tone shaping plugs as needed.
 
The majority of my vocal tracks are recorded with a pop filter, singing close in to the mic, and no plugs in the track. After the fact, the EQ curve is generally still left fairly flat with a slight dip around 800-1000hz. 
 
For the best results, keep it simple.

This is sensible, thanks.
I do sing quite close (Old bar band SM58 habits) but I do have the low end off the SM7b AND on the input COSM effect on the BR800 controller so I'm probably overdoing it.
And yes, overdoing it on plugs too.
I'm sensing a pattern here. Lol
Thanks again.

Hey, Glenn, Like the others, I would be concerned about the mic or positioning if the sound were recorded sounding like you describe; however, in answer to your question, from your description, I would think you have way too much 500-800 HZ—hard to be more exact without hearing it.  Really muffled would be at the lower end of that range.  Cardboard box would be at the higher end.
 
Is it possible that the mic input is way too hot, or that you have mic pad on the mic (-5, -10) that you need to turn on?  If you were recording way too hot for the mic or input channel, you could be getting major clipping and distortion that could be described that way.  Every time I change mics and every time I change singers I need to go into my mic preamp settings and readjust things.  With a condenser mic, I almost always have the -5 or -10 switch on.
 
Tip: Even if you meter is showing the same level, a mic sounds different in a preamp with a -20 pad on and the level up than it does with the -20 db pad off and the level down.  Someone will probably argue with me on this on an electronic basis, but I can hear it: more distortion with the pad off even if the meter says the level is the same.
 
 
UPDATE: Went to the site.  I think you mean the Reggae song.  I had a lot of trouble with soundclick but I heard a little.  It does sound awfully 800ish but I am sure you will want to check all the causes listed in this thread.  Did you say anywhere what kind of mic this is?

Thanks Konrad
It's an SM7b