I vaguely remember the question of using other tube types in the RM200 coming up in a Music Reference thread a few years back. If I remember correcty, while the RM200 was built around the KT88 tube, it could use most (if not all, just can't remember) of the other standard octal power tubes. Of course, you'd have to rebias. You'd have to do some major searching to find that post, many of the threads in this circle go off on tangents irrelevant to the topic, though very important nonetheless. Also, this doesn't necessarily mean that the RM300 could be rolled similarly. You'd have to contact Roger directly for that answer.
Wow! SE monoblocks! You must be made of money!!!!