The Session 400 is a 200W solid state amp (where the 400 in the name comes from, I'm not sure), and its biggest selling point for pedal steel seems to be loads of clean headroom. In the HD500, I think the amp model with the most clean headroom is probably the Twin, but you probably need to get into the DEPs to maximize that. That would be where I'd start, anyway. Are you looking for something that's super clean? You might actually be able to get away with not using any amp model. Or you could put a dry path in parallel with the amp model. It really depends on the type of amp tone you're after.