I have owned both, the 300 has the edge on clean headroom. The 300 was noticeably heavier to lift and carry. My Mk3 was bought new in 1983/4?, it is still going strong, although not with me. The only failure was the speaker about 5 years ago, the glue holding cone to frame gave up with age. Bought a 15" Eminence from this forum and it lives on.