BAT VK-31SE with 6H30-DR NOS tube

Apparently Victor has a nice stash of the NOS DR tubes and provides them as stock with the flagship 51SE. Obviously the DR tubes will not make the 31SE sound as good as the 51SE. The 51SE is IMHO substantially better than the 31SE but it is up to the buyer whether that difference is worth the extra cost. Get some of the lower priced DR NOS tubes(you only need 4) and see if they improve your 31SE 'enough'. If not, they will not be wasted and you may end up considering the 51SE.

As a former 30se owner, I would be careful when you evaluate the "upgrade" path through BAT. They have some of the most expensive product upgrades I have found, (Ayre is a great example of how it should be done), and when I compared the 30 and 31 there was no where near the difference that the price of the upgrade would denote. If you are looking for more performance you should really just look at a 51se if you like the BAT sound. I gave up and got an Aesthetix Calypso which I found to be a better overall preamp that you can also tube roll with fairly inexpensively because it uses a more common tube set.