I have the stereo version Stealth Pro, and the larger SD700, so not really a direct comparison. I feel that the SD is a more robust/better made product. The fan is not something I ever hear.
The ISP is way lighter, but that's mostly because the power supply (large laptop size) is outboard, but also because the housing is thinner/lighter.
The ISP is class A/B and the Seymour Duncan is class D. I like the fact that the SD is using the Icepower modules just Like Kemper does.
They both sound good, no issue there, but my personal vote is for the SD.
I think you'll be happy.
Thanks a lot. This definitely make me feel content.