The 550ex is also more expensive than the DM Stratos.
IMO, the FetValve is probably not a good fit for all systems - the Odyssey is a warmer sounding amp, and would be better in a system that is a bit lean in character. But for neutral or warm sounding speakers, the FetValve is going to be very hard to beat, especially if you need a lot of current and power deliver to the speakers (I would love to hear the 550ex with a Dunlavy SC VI).