Are you looking in a certain price range or just looking at what's out there?
The ISF feature didn't sell me at all. It made a difference, but I think the tone of the amp was based on an EL34/Marshall sense of high gain. I personally wasn't too impressed. I tried the HT and at least one other.
Just for reference. The major high gain heads that I've owned are Peavey XXX (hated it, never felt balanced), 6505+, Mesa Dual Rectifier, and I currently own an Engl Powerball. If I had really known how much a gain boost in front and an eq boost (I use a BBE sonic maximizer) in the loop can do for anything in the 5150 range, I probably could have saved a lot of time and money.
I'm actually buying another 5150II tomorrow. $650 with new tubes and a flight case. They're so damn cheap right now and easily modded. If price is the issue, you won't go wrong with a 5150/6505. They're time tested beasts and they can get LOUD.
If money isn't an issue, and you are looking for an "American" or "modern" high gain, then Engl or Soldano are my votes.
I want a low wattage head so thats why I'm looking at the ht20. I didn't want to get up past the $1000 range. The Dark Terror is a possibility too. So soldano and engl are out. They are all too much wattage anyway. I'm gonna look at Krank Jr.