Gain Ratio Relative to
Tube   Gain   12AU7   12AV7   12AY7   12AT7   5751   12AX7
12AU7   19     1.00    0.46    0.42    0.32   0.27   0.19
12AV7   41     2.15    1.00    0.91    0.68   0.58   0.41
12AY7   45     2.37    1.10    1.00    0.75   0.64   0.45
12AT7   60     3.16    1.46    1.33    1.00   0.85   0.60
5751    70     3.68    1.71    1.56    1.17   1.00   0.70
12AX7  100     5.26    2.44    2.22    1.67   1.43   1.00

So, reading the table, the 12AU7 has 0.42 the gain of the 12AY7, and inversely, the 12AY7 has 2.37 the gain of the 12AU7.

In general, if you find your amp distorting too much, try moving to a lower-gain tube. If the amp sounds weak, try a higher gain tube. It's all a matter of what sounds good to your ears.

That said, most 6-stringers like 12AX7 tubes, since they are higher gain and can more easily overdrive succeeding tube stages. This is also why NOS 12AX7s are so hard to find and expensive, there are lotsa guitar players out there using them up. On the other hand, 12AU7s are not very popular except for tube Hi-Fi, and are fairly plentiful and not too expensive. For steel, 12AU7s can be useful if you want to tame a hot circuit. Myself, I often use 5751s and 12AY7s in old Fenders for steel, like in my Deluxe Reverb or Dual Showman Reverb.

This is decidedly one of the major advantages of tube amps - you can easily change the sound and distortion characteristics of your amp by doing intelligent preamp tube substitution. Obviously, this doesn't apply so easily to power tube substitution.

On the question of brands, I've certainly tried a lot of different brands - again I argue it depends on the circuit and ones preferences, not to mention the price. In old Fenders, I tend to like the American-made JAN Philips/Sylvania or GE tubes if they're available. The GE tubes seem to have a slightly softer, more compressed sound overall to me, where the Philips/Sylvania tubes overall tend to be a bit less so. But honestly, these things vary from run to run and from tube to tube, making generalities very difficult, IMO.

Right now, Angela Instruments has 80s Sylvania 6189W (industrial 12AU7) very reasonable (< $10 per tube - I'm probably gonna grab some while I can), RCAs (also a good choice) twice that, all the way up to Mullards and Amperex Bugle Boys, which I think are over-rated and $50 a pop. I think 12AU7, with its low gain, is the least sensitive to brand. I'm much more concerned with microphonics. Since brands like Mullard and Amperex are getting in very short supply, I think it's much more likely to get a dud. Let's face it, we're getting to the bottom of the barrel.

On 12AY7s, that's getting tougher, they're used a lot in Tweed Fenders, so they're getting harder to find. I have a GE 12AY7 that Angela sells for $25 apiece now in the front end of my Deluxe Reverb and the GE 5751 that he sells for $27 apiece in my Dual Showman Reverb front end. I really like the sound, and I bought these when they were less than $10 apiece.

But I'll try different brands if they're reasonable, and I can almost always find an application for them if they're good examples, not microphonic, etc. On non-12AX7 preamp tubes, I still think it's better to use NOS, but that will probably not continue much longer as the supply of good ones continues to get used. I honestly believe it's ones preferences and the circuit that dictate what tube should be used, and there's no one best answer. I don't use outboard tube preamps, so I can't comment there at all. Finally, all of this strictly IMO. I already made my caveat that I think it's more important to get a good example of a NOS tube than some specific brand. YMMV.

Brad