If you choice is between the Sunfire Sig. and Parasound 2205 I would definately go with the Parasound. Not only will you save $600 to $800 but it will more than likely sound better. The Sunfire Sig. is a good amp but it never distinguished itself when I tested it with the Parasound, Aragon, Classe, ADA and EAD amp shoot out. I did this over a week and my conclusions were as follows: 1) EAD PM2000 2) EAD PM1000 3) (Almost tied for second) Classe CAV-150 4) Aragon 8008X3 5) Parasound HCA-2205a 6) ADA PTM-6150 (this is a very good amp that if I could save $200 or more on the Aragon or Parasound I would select) 7) Sunfire Signiture Grand.
I will say that the Sunfire did have different speaker outputs and one did sound better for music than the other (I can't remember if it was the current or voltage outputs) but against the other amps listed it never won out, just tied in some areas.
If you were thinking of spending the $1,750 + for the Sunfire Sig. I would definately look for a used EAD PM1000 that can be had for about $1,500 to $1,700 on the used market. If not then the Parasound would be a great choice and you would save some money to boot.
As far as THX is concerned I would never use that as the deciding factor, your ears are the best certification around.
Good luck
Mike