I just received my Iconic 80 watt head last Friday.
I'm running it through a 1960B cabinet with 75's. This thing puts out an incredible heavy tone. Not for everybody but it suits me. The clean channel is tonally very rich and smooth.
I'm using my GT1000 4CM which takes it to another level...
Put it in context, when the boost switch on the JCM1 is engaged I think it tends to compress and muddy up the amplifier. My 2555 is a totally different beast.
The midi switching with the JVM is fantastic. I use my Boss GT1000 with the four cable method through the loop and just couldn't imagine any other set up that would surpass this one.
I don't use any of the GT's pre-amps, all overdrive comes from the amp itself.