Very little I suspect apart from volume, depending on whether the 400 has 6550 or 6L6GC valves.
[url="http://en.wikipedia.org/wiki/Mesa_Boogie"]http://en.wikipedia.org/wiki/Mesa_Boogie[/url]
[i]Demand for a more powerful amp led to the development of the Bass 400, which replaced the D180 in mid-1985. It used six 6550 power tubes to push out about 250 watts; however, the cascading-gain channels were replaced with two separate hi- and low-gain channels. Mesa's supply of 6550 tubes was cut short in the late 1980s and they were forced to sell the 400 with 6L6GC tubes, reducing its output to that of the D180. The solution came in early 1989 in the form of the Bass 400+. It was the same as the 400 in most respects but was powered via twelve 6L6GC tubes for nearly 360 watts of power.[/i]