Regarding the 12-- series tubes, I too prefer the 12AU7. The 12AX7 and 12AT7 tubes are meant for high-gain applications, such as phono stages and mic preamps. In this application, they are excellent, but in a linestage application, where voltage gain is secondary to current buffering, these tubes are not ideal.
The key is to look at the tube curves for the tubes in question. You will find much greater linearity in the 5687, 6SN7, 6CG7, 6H30, 12AU7 and ECC99. Compare this to the curves for the 12AX7 and you will see what I mean.
The other thing to consider is the amount of current that the various tubes can deliver, as well as factors such as plate resistane, transconductance, etc.
The popularity of the 6DJ8 is a bit of a mystery to me. It is a medium gain triode and has decent but not great specs. It IS however, readily available and mfd. currently by different companies. I don't think that it is a bad tube, just that it isn't as musical as others that I prefer.
Dan