I own both a MF2300A and a MF2500A which are great amps! As for hum, if you have a ground loop problem, CJ amps will let you know it. Most people cheat around it by floating the ground. I also had an opportunity to compare Plinius amps (which are really nice) to Conrad Johnsons and the CJs blew them away! And I'm not a biased (no pun intended) Conrad Johnson dealer...
Take care
Take care