It all depends on what power transformer is in your IIC+. The 100 and X101 have the 450-460 plate voltage. The 105 power transformer has a plate voltage of 485-ish. The Mark IV has a plate voltage of 460V +/-, if I recall correctly. Why do you ask?
I'm in the process of buying my first NOS tubes (power & preamp tubes). The vendor I talked to wondered what plate voltages they would be operating at. I very exite! 8)