It's probably worth asking why do you want (a fairly powerful) amplifier for iems?
In my experience probably not the best. My unit may be atypical but I have a hum that is audible on low gain and no music playing with my Grado SR60s on every tube I've tried. It's not volume dependent, but I'd say it's still usable with many of them. With HD580s I don't notice it at all on low gain with most tubes. I would think most iems are more sensitive than the Grados and sitting right in your ear canals, so probably not the best for a tube or hybrid amp in general.