I was wanting to do the same thing with my 4jr's and I emailed a few people on this site and got great advice and sold the m-300's and bought a Moscode 401hr which was exactly what the doctor ordered. It's also fun because you change the sound easily with tube changes. This is one fantastic amp with the 4jr's. Sorry I couldn't answer your question but I thought you might want to look into this.
Good luck
Gary
Good luck
Gary