You could get 4X Sundown 3KD's used or Crescendo 3K's new for the price of 2 3500's FYI. I'd be willing to bet you wouldn't hear the difference between a pair of 3K's and 3500s. In theory the difference between 6000 and 7000W would be under 1/2 db on a meter. Hardly worth a grand to gain .5db unless you're well into the 160s allready IMO
I used a Stetsom 7K for a while and it's pretty bare bones for features but put out a shitload of power in a pretty small footprint. I know a few guys using the AB 500.1 and it seems very strong and reliable.