So one of the questions is why the 6C45PI, this may have been answered or discussed but havent seen it?
When you look at triode characteristics and consider voltage gain, transconductance (think output current), and internal resistance, often times you can really only optimize one of those characteristics, or potentially push the envelope a little bit on two of them, but not all 3.
The 6C45 is a bit of a special tube, as it has pretty high voltage gain, very high transconductance, and low internal impedance. The 6C45's limiting factor is that it can't handle high plate voltage. This allows us to just use one 6C45 per channel (unlike something like the SEX or Crack, which develops voltage gain in the first stage, then uses a second stage with lower impedance/higher current to drive the load).
Were other tubes considered in the design of the Mainline?
There was the Smack headphone amp before the Mainline that used the 5687. We found that it did not have sufficient gain under some circumstances, especially as source voltages have dropped over the last decade or so.
A Western Electric 437A will work in place of the 6C45PI, but at quite a cost.
There are some random pentodes that can be triode strapped and reworked into the Mainline schematic, but we'd recommend sticking with the 6C45.
