I'll add to this thread the the 31 and 31se are SIGNIFICANTLY better than the 30SE. Ditto for the 50 vs 51SE. BAT is one of the better high end companies. You won't be making a mistake buying from them. Tom has the best advice, if you can try at home, do it. Just be prepared not to return it :).
The 31SE is a little more extended, more dynamic, better stage, better details. The 31 is a little sweeter and warmer depending on what 6922 tubes you use.