(1) A 12AU7 is identical to a 6CG7 electrically and in the circuit it will function identically. However, there is a different heater pin out. So if you are substituting you need to make a change. In addition, the 6CG7 draws twice as much heater current so be sure that this will work in your circuit. (This may be one reason it is not as popular as it should be.) If you need help write me via http://www.oldstockaudio.com/More/contact_me.htm.
(2) The 12BH7 requires no circuit modification to substitute for a 12AU7, but the plate resistance is a little lower (5300 vs 7700). Thus, even though it has a lower u (or amplification factor), the gain may be slightly higher depending on the circuit design. [This is mainly true for circuits that use an anode resistor load.] It will also draw a little more current, but the 12BH7 is a beefier tube and can handle it. For most circuits this should be well within the tolerances of the design, but you can ask the manufacturer to be sure.
With respect to audio reproduction there is a fairly large difference between these three tubes. The 6CG7 (or 6FQ7 which is the same exact tube) is identical in internal construction to the 6SN7. The 6SN7 has the reputation of being the most linear of the medium mu triodes and the 6CG7 follows suit. Also, the 6CG7 line is remarkably similar in audio performance across various manufacturers of the same vintage or age. It is a good tube for early or middle stage amplification and has the added bonus of a grounded shield between sections, which reduces hum and increases channel separation.
The 12BH7 can lend a wonderful life to the tone that sounds great for jazz or vocals. For audio listening, this tube can temper the harshness of CD's. There is, however, a wider variation in the sound of 12BH7s than 6CG7s, so some tube selection or rolling is in order. You can find very neutral 12BH7s (watch for a coming blog comparing them) . If you don't want to modify the circuit and are unhappy with your 12AU7--this is a good choice.
The 12AU7 family has the widest variety of tonal characteristics which is probably why this tube is rolled so often by audiophiles. I personally find most 12AU7s a little tinny in comparison with the 12BH7 and 6CG7, but there are exceptions. The RCA clear plates from the 60s are more like 6CG7s in their sound, for example. There are lots of blogs and posts about the difference between 12AU7's so I won't waste your time repeating everything. My favorites are Mullard and Siemens, with RCA clear tops and Tung Sol a close second. A lot of people like Amperex, but they work best in a system that is not forward or bright as they add life to the midrange. My Lowthers are already bright in the mid-range, so I stay away from Amperex 12AU7s.
Part of the fun and magic of tubes is rolling them when listening to different types of music. When you do, we hope you will consider our vintage tube offerings at http://www.oldstockaudio.com/ .