I had a B6100 with Turf tires on the back and R1 tires on the front. I had the rear tires loaded with WWF. That gave the tractor lots of ballast and lots of stability. I'll second (or third) what others have said about Loader technique and "peeling" a full bucket off of a pile. You need to keep the RPMS up to effectively work the Loader. My tractor was a gear transmission so I set the RPMs with the hand throttle and controlled tractor movement with the clutch, brake and gear shift. I always used 4WD when doing loader work and since it involved a lot of turning, I never used Dif Lock.
I'll also second/third the advice about dropping the loader down low Immediately after you fill it. Your next maneuver is going to be accelerating backwards and turning simultaneously and if you've got the loader up at hood-height or above you're setting yourself up for a tip-over.
I have the BX2230 also, with turfs. My tires are loaded with washer fluid that adds about 90 lbs per tire. Great addition for stability on hills. The fel is rated at 211 kg which is 465 lbs. I've put 580 lbs in the fel and raised it waist high at just over idle w/o any problem. I didn't go higher because the rear end was already light. When scooping snow I've been on 2 wheels pushing the front end in the air and squawked the tires.
I have no idea where my relief valve opens; never measured it. Sitting about 1070 hours now, I'm 2nd owner.