You're not getting deluged with replies, so I thought I'd chime in.
I also own a J2, in my case driving my horns. It's everything Photonman said. If you buy from Mark at Reno Hi-Fi (as I did), as far as I understand things they will let you try out the F4 for a certain period, and return it if you're unhappy.
I wanted to make sure you understood that the F4 is a buffer, and has no voltage gain, so it requires a fairly hefty voltage drive to reach full power.
Francis
I also own a J2, in my case driving my horns. It's everything Photonman said. If you buy from Mark at Reno Hi-Fi (as I did), as far as I understand things they will let you try out the F4 for a certain period, and return it if you're unhappy.
I wanted to make sure you understood that the F4 is a buffer, and has no voltage gain, so it requires a fairly hefty voltage drive to reach full power.
Francis