I own the 3i. I use Sovtek Rocket logo 6h23 tubes and either Pope or Visseaux 6v6 power tubes. From what I have read and heard with my own ears, the 3i is too good of a value to look higher in the BAT preamp line... unless you have the money.

If I had the money, I would search for a used 51SE or just a 50 or 51 and have it upgraded to the 51SE. The vk5 is, from what I have read, sonically similar to the 3i. It just uses balanced inputs and outputs everywhere with no use of RCAs at all.