For general endurance enhancement I would second Iron’s suggestion. However if you wanted strength and agression increase without too much extra weight then I would look at masteron (mma fighters seem to love this compound). If you wanted speed and strength increase (with a little agression to boot) then look at winstrol (there is a reason Olympic runners used this until the tests could catch them). Now with any DHT, which mast and winstrol are, you can have joint issues. The joint issues are basically from there not being enough estrogen in your body (a lot of guys run these without any test trying to cut). Estrogen actually has a natural anti inflammatory and pain relief quality to it thus if there is no test to convert then there is no estrogen. So a lot of those achy joints on winstrol are from it not being run correctly with test.
Ultimately the question to answer is do you want to go up a weight class?