Wheel bearings are good and front end components are tight. But riding at speed the handlebars keep turning left and right making it feel like I don't have good control? Any thoughts?

 

Either your tracking is out or you have the wrong choice of tyres. I didn't believe tyres could make such a difference until I got a Suzuki 450 for repair that had the opposite fault, it felt like the diff lock was on as soon as you went into 4WD. The owner wanted wide tyres anyway, and I expected it to be worse, but no, once the wide tyres were on, the steering was much lighter. Other possibility, the machine has had an accident, then the whole geometry could be out.

 

The Grizzly 660 isn't the easiest machine to steer, I can attest to that. The EPS first introduced in the 700 made a world of difference in ease of steering and handling. I love my 660 but its not perfect by any means. Check tire pressure all the way around. That's something that many overlook, one or more tires might be underinflated. I run 5 psi on all 4, works pretty well for me. Low tire pressure can make the front end seem a bit unstable.

 

Maybe the front tie rods are out of adjustment. and a tire or tires is toed out or in .

Blasting down a gravel road at 50 mph , power steering would have no advantage ,infact it may give you less road feel back when speed riding.
Power steering is more for slow rough trails where the front wheels are in ruts and getting knocked around by boulders.