Got a good chance to A/B both today! I'm pretty convinced that the 400 is the more efficient amp by far, because they both top out at the same maximum output, even though both are loaded with 6L6's. Tone-wise, I found that the 400 is more tightly focused and has a more pleasant, full-sounding growl, whereas the 400+'s distortion is pretty harsh. However, the 400+ does have a tad more clean headroom, but I bet if I drop some KT88's in the 400, it would be nearly the same. Strangely enough, I actually prefer the sound of the 400. I haven't even modded it yet (12AU7 driver, adjustable bias), and it sounds great. I can't wait to gig with it!!
One other neat thing about the 400 is that it's nearly as short as my old 800RB, so it fits in shallow rack cases a little easier. Only thing I don't like about it is the fixed power cable :[
As for that master push-pull, I still have absolutely no clue. When in, it sounds normal, but pulled out it got a little punchier, but that's it.
I also found that this is one of the last 400's out of the factory..... I called Mesa and asked how many of them they made; he said that they only made 1200 of them. He also said that they transitioned to the 400+ in the very beginning of 1989 (Jan or Feb.) and my 400 was made in 1988..... so I guess that explains why it's got the 2 ohm taps off the transformer. Maybe the push-pull master "punch" switch was a very late idea that was abandoned on the 400+???
