You will not be let down by the JJ's...I completely retubed my II Non-Pro with JJ 6550 and some NOS GE tubes for the preamp...now the amp is so articulate, I can almost see the notes I play. I love it. This one was on my bucket list, and I adore it.
1+ on the JJ's. I've not been disappointed with any of the amps I've repowered with them.
FYI. JJ KT88's will fit in the IINP but there is only about 3/16" clearance between them. I was concerned about them getting too hot being that close together but after doing some testing I realized that they didn't run any or much hotter than the 6550's in my other IINP.