The real issue here is that your gear is using a 12AX7 and 12AU7 in the first place. Most of them suck, plain and simple. There are only a few of them that sound good. I know... Broad statement, based on my findings with them. IMO, YMMV...
The Amperex tubes are good, but I find them a tad soft for my tastes. They emphasize the midrange, particularly the upper midrange more, and are softer at the extremes. They have great presence, image well, but this could easily tip the balance of your system.
I'd go for regular Mullard tubes before Amperex myself. They at least get the balance right.
The only other 12AX7 besides the 10m I would consider is a 5751. GE and RCA would be my top pics. Either grey or black plate, 2 mica or triple mica. They are all about the same to me, but really good nonetheless. These don't last as long, but will give much of the 10M performance for less dough. They also do not work well in all gear that requires a 12AX7. They will either work wonderfully or not. Bottom line, the 5751 is a fantastic tube, if it works well in your gear.
12AU7 tubes, with a couple acceptions, suck. Just not a very linear sounding tube. The 7318 stands well above them all. I'll even say that the 7318 is one of the best small signal tubes you can use. If one has the headroom on heater current, your closest alternative would be a 12BH7 or ECC99. These will tax most gear though as they draw double the current, or more, that the 12AU7 does. With these big plate tubes, good luck finding some quiet ones also.
I like linearity, speed, big dynamic contrasts, focus, 3D holographic imaging, bass slam, and vibrancy. No glare, spit, graininess, or flab. Simply put, I want it crystal clear and like live music. Well, like I envision live music should be at least ;)
The 10M 12AX7 and CBS 7318 really nail this better than any other of these I have tried, and they translate the same to every piece of gear I have tried them in that uses 12AX7/12AU7's. They just so happen to be very tough long lasting tubes that are very rare and pricey. Damn the luck :)
Best thing? Find gear that does not use a 12AX7 or a 12AU7. Lots of better tubes out there that are much cheaper than the best 12AX7/12AU7. How's that?
