If I had to define it, I'd place Moontower into the prog metal drawer as well, whereas both Crimsons are more death IMO, and Nightingale.... ummmm simply prog?
Threshold is not your average prog metal band though - they combine high technical skill with heaviness and soaring melodies in a way that not many others do.
I really love Dead Reckoning, probably Threshold's strongest release to date, and Dan, well, short as his parts may be, adds a great bite to those two tracks he's on.