First time reading this post, excellent review and comments regarding the amp and would find it helpful if and when I decide to scale things down.
As for the JAN/Phillips 12AT7 will be a warmer tube than the Mesa 12AT7 if used as suggested. It will effect both channels such that you may desire the gain characteristics of the standard or stock 12AX7 tube.
As for the EL84, I would recommend the Mesa version since some of the other brands may require bias changes as this particular tube can be quite different across the brands. It is somewhat similar to the larger tube EL34 as they are both power pentode tubes, but the tone and gain characteristic of the EL84 is quite different than the larger bottle cousin. I do have an EL84 based amp I use for practice that I keep at my friends house but that amp also has an adjustable bias (Carvin V3MC). Bottom end is no question and there is plenty of signal at top and bottom to satisfy anyone using a 6, 7 or 8 string guitar or alternate tunings. The signal is going though a 12AX7 tube so why should the size of the power tube matter in terms of frequency response? They are a bit different than EL34, 6L6 or 6V6 but in all respects they are just as good but lower in power rating.
I have considered many times to get one of the smaller Mesa amps, and I did just that.... but it has a quad of 6L6 tubes vs a quad of EL84s and weights about 14lbs more than the Mark V:35. JP-2C, probably louder than I need but why not? One of these days I will have to try out the EL34 Based Mesa amps.
As for the JAN/Phillips 12AT7 will be a warmer tube than the Mesa 12AT7 if used as suggested. It will effect both channels such that you may desire the gain characteristics of the standard or stock 12AX7 tube.
As for the EL84, I would recommend the Mesa version since some of the other brands may require bias changes as this particular tube can be quite different across the brands. It is somewhat similar to the larger tube EL34 as they are both power pentode tubes, but the tone and gain characteristic of the EL84 is quite different than the larger bottle cousin. I do have an EL84 based amp I use for practice that I keep at my friends house but that amp also has an adjustable bias (Carvin V3MC). Bottom end is no question and there is plenty of signal at top and bottom to satisfy anyone using a 6, 7 or 8 string guitar or alternate tunings. The signal is going though a 12AX7 tube so why should the size of the power tube matter in terms of frequency response? They are a bit different than EL34, 6L6 or 6V6 but in all respects they are just as good but lower in power rating.
I have considered many times to get one of the smaller Mesa amps, and I did just that.... but it has a quad of 6L6 tubes vs a quad of EL84s and weights about 14lbs more than the Mark V:35. JP-2C, probably louder than I need but why not? One of these days I will have to try out the EL34 Based Mesa amps.