The Heed CanAmp is very cabable, as is the M^3, a bit more so than the Darkvoice 336i I felt, but I haven't heard a 332. I vote for stretching your budget whenever possible, so my true vote goes to the Wheatfield, even though I haven't heard that either. The K340 is one of those cans that will make you itch until you get your amp just right, and thus far I haven't really experienced that bliss. The Heed was a nice surprise though, and makes a great option coming in well under budget.
Also, just looking at those Wheatfield pictures, that is the amp you want. So rugged and powerful. It will make your K340 feel like a woman.