I've got a NH 634, which is a 4x4 round bales. Run it behind a JD 5065E which reportedly 53 claimed PTO HP (58 tested by TractorData.com) and have no issues. Personally, I believe weight of the tractor is a more limiting concern than PTO HP. Neighbor has a Hesston 540, which is also a 4x4 round baler. He runs his behind a Kubota MX5100 which is 44 PTO HP with no issues. My NH has a bale kicker on the back so you don't have to back up to dump a bale like the Hesston (so that the door will have room to close when you pull forward). If I could have found a decent 4x5 baler, I would have went that route. Everyone seemed to want that size and they were priced accordingly. Observation without back-up facts, the Hesston 540 appears to clog up quite a bit more than the NH634 if you're in big wind rows. Both of them are vintage mid 90's. Both are belt driven balers which make tighter bales than the early chain driven round balers. Around here, you can pick up a chain baler for pretty much next to nothing.