I have some short thoughts on my 4 sets of tubes that someone might find helpful.
6ZH1P-EV - (aka VOSHOD 6J1P-EV) These are my favorites. Great instrument separation and detail. Tight bass. Nice airy sound without sacrificing weight and depth. They have performed great with any pair of cans that I have thrown at them from Grados to Senns. They are the perfect all arounder. They are at their best with Rock, Funk, Jazz and rap and get thumbs up from me for Soul, R&B, Country and classical.
Tung Sol/National 403B - These would be my favorites if I had a better source. (I have a new DAC on the way). My current set up is just my MKIII plugged a mid level soundcard. The Tung Sols ruthlessly expose my source for what it is and everything sounds compressed, dark and there is a distinct lack of instrument separation and detail. However, they sound awesome with my standalone CD player which is by no means Hi-Fi. I am anxiously waiting to hear these tubes with my new Zero DAC followed shortly by a discrete opamp. They have a very silky sultry sound that still marries quite well with my K-501s despite showing the obvious shortcomings of my setup. However, I am still a bit underwhelmed at how they sound with Sennhieser cans as they lack "air" and sound too full. I am hoping that a discrete opamp will give them some much needed breathing space.
Mullard 8161 - Awesome. These babies and the HD-580s must be soulmates. I cannot really describe how great this pairing is. This pairing handles everything I throw at it. Excellent detail and separation.
They are a tad too airy though which is great for darker Senns like the HD580s or my old pair of DR150s but with cans like Grados, Open Air Audio Technicas, and the airier AKG offerings the music lacks weight. They sound ok with the DT 880s but the sibilance was cranked up to fatiguing levels. With the K-501s the music virtually floats away from your ear. Thus I rarely keep them in because my favorite cans are the k-501s. They will probably fill in more with a better source but I don't expect their overall character to change much. A better DAC will more than likely make them more airy, depending on what opamps you use of course.
These are probably THE choice for all Sennheiser cans and probably the K-601s (I have heard these are darker and more rolled off than the 501s and 701s).
Mullard 6AM6 - My least favorite by a wide margin. They don't sound bad and I used to be quite high on them but the more I have compared them to other tubes the more I dislike them. They are thin sounding offering less air than the Mullard 8161s but at the same time offering less musical weight and fullness. I prefer them slightly to the stock tubes but that isn't saying much.