The HR+ is basically the SLO with standard parts, so it's a crunch channel and a lead channel in one amp. The HR is a single channel amp that's Mikes Marshall mods right into a home built amp: basically, it was cheaper for him to build them from scratch rather than moding new amps.
I've played the HR+. They're really good, and in fact the difference to the SLO is fairly small (though noticeable). I haven't played the HR, but understand them to be a bit more Marshall-esque in sound.