I've owned the MW Sony 5400 for quite a while now and for me, it's a keeper. I also have a huge stash of NOS 6SN7's I've accumulated over the years while looking for the best(for me) tubes for my Atma-Sphere MP-1 pre and MA-1 mono's; combined, they use 20 6SN7's!
I use the Mullard rectifier and like it a lot, but as mentioned, it will add a bit of warmth. If you're looking for more "crispness", I'd probably just stay with the stock tube or try some of the current offerings.
As for the 6SN7's, I'd pick up a couple GE 6SN7 GTA's or GE 6SN7 GTB "coin base". They are seriously good and quite inexpensive and they'll give you the open, airy extension in the top end you're looking for. JWM has heard them in both his MW Oppo and his MW Sony 5400 and he too liked them a lot.
Another option might be NOS Tungsol GTB's(not the black glass version). They're also inexpensive and offer a leaner, cleaner sound with great extension on top. As mentioned, there are a few Sylvania "W" variations that may do the trick but they tend to be a bit pricey, especially the nickel base version. The Kenrad VT-231 is also a very nice tube but they often tend to be microphonic so buyer beware.
As for the MW Oppo vs MW Sony debate, JWM and I, while good friends, agree to disagree on which to go with. Of course, there's no wrong choice cuz they both sound exceptionally good but I prefer the Sony.
I spent a lot of time in the Modwright room at this year's Rocky Mountain Audiofest and Dan did several A/B/C comparisons between the stock Oppo, his tube modded Oppo, and his tube modded Sony. EVERYONE agreed, including Dan, the Oppo is warmer and fuller but the Sony is more resolving. Everyone also agreed, the stock Oppo ain't bad but either modded unit mops the floor with it!
After several hours of listening to JWM's modded Oppo vs his modded Sony, in his system of all VAC equipment, I came away with the same impressions. We went back and forth between the two players, and by the end of the night, we pretty much agreed, the Oppo sounded warmer and fuller, the Sony was leaner and cleaner. The Oppo sounded better on mediocre recordings but I thought the Sony made good recordings more exciting and believable. To my ears, the Sony is more resolving, open, transparent, and extended on top. As for which has the best bass, I'd call it a draw.