The J2 actually yields an even broader soundstage than my 300b monos and during the summer heat in CA, it is a welcome change with no lacuna in its performance. It is remarkable amp like all Pass Labs gear and sure to be long lived.
I have the J2 and hope to use it for the first time when my new Joseph Crowe speakers arrive. From what I've read it gets even hotter than my First Watt F4 amp. Presumably, it puts out less heat than your 300B monos, yes?