I'm with you on the improvement they bring to the HD 650 - a very spacious and clean sound.
The MK IV can be used with the EF92, CV131, WE403A/B, GE5654, M8100, CV4010, EF95 and 6JI families of tubes, which gives a lot of options RE tube rolling (although I haven't rolled any tubes in my KM IV yet). The MK III is compatible with exactly the same families of tubes as the MK IV, so you should be able to glean plenty of relevant information from MK III tube rolling thread.
I don't know if a MK IV tube rolling thread exists, perhaps you should start one . I'm sure everyone with a MK IV would like to hear your impressions.