In this world of high priced gear, most shops need to be very selective of what they stock. You mentioned the top three adv/expert midfats on the market. It isn't just a coincidence that these skis are found most places to demo, as well as most shops (at least before they sold out!). If you think that the Head might be somehow better than these skis, I suggest for the right person they might be, but for the mass market, no.
Ski any of those that you mentioned, and don't look back. I have skied them all, and they are all wonderful all-mtn. skis.
Here is my opinions of their strengths:
X-Scream - The original midfat master. Probably the best short turner of the bunch. The 187-cm is probably about as long as you need to go unless > 200 lbs.
Volkl G31 - The race car. Loves big fast turns. Needs a very skilled pilot. Bigger guys seem to like these. Skis long, so again, the 188-cm length is best for average weight experts. Probably the best of hard-pack and most difficult in bumps.
Mod X/Mod X Pro - Nice ballance between the two above. Seems to handle speed better than the Scream, but is a bit more damp. Does extremely well in soft snow. 188-cm length for most guys. Not stiff, but surprising grip. One of the easiest midfats to ski, IMO.
I couldn't decide so I got a Mod X Pro and a G30! They both rock everywhere, and unless the price is just too good to pass up, I would get one of these three and forget the Heads.