For those of you with an NTK, I just wanted to share that I upgraded the tube in my Rode NTK from the stock to a mullard and...it makes a big difference. Mellows out (somewhat) the high end sizzle, and gives a much fuller low-mid response, real...tubey. I am extremely pleased with the $79 I spent.
http://store.tubedepot.com/nos-6922-mullard.html