I too have a 300T and use and old Sunn 2 x 15" cabinet that has 2 JBL K-140's in it. I am not absolutely sure about my cab's power handling capacity, but I am told these speakers can hanlde 150 watts each no problem- 300 watts using the pair. I have never had a problem and it has always sounded kick-ass.
I have heard that in order to get the best tone from amp/cab set-up, the amp should be matched with a speaker cab that is at or a little more than the amp's power capacity. Mabey this is why my set-up sounds so nice. I do watch out that I don't max my 300T out though. Usually set at 3-4 on the volume - and the same or a bit lower on the master volume. Don't want to blow the speakers since they are approx. 30 years old you know. Your 8x10" ampeg cab at 300 watts should be fine so long as you don't push the amp too hot.