Early preamp breakup … 5879 preamp tube … PI … ?

Hello all! I am hoping to get some discussion going on some aspects of some classic amp preamp configurations. There are two aspects that I am interested in below.

1. Use of preamp pentode for early breakup but more specifically the 5879 used in early classics like the GA-20 and AC30. Also used in channel 1 of the /13 RSA23 and FTR37. The GA-1RVT uses a different pentode in the preamp but has a wonderful growl. I can understand the early amps utilizing what we now consider more exotic tubes because in those days everything but toasters had vacuum tubes, but it is intriguing that /13 and other modern builders went out of their way to use NOS 5879 tubes. If pentodes are used in the preamp it seems they are always used as the first preamp stage. Can they really be the source of the breakup we hear, or is that the next stage which is either a 12AX7 type stage or a phase inverter like the one used 1960 AC-30, RSA23 and FTR37?
2. Is the AC30 phase inverter configuration a source of early breakup? In researching use of preamp pentodes, it occurs to me that the main breakup character is probably coming from the next gain stage, not the pentode. I have seen Tim Pierce demo channel 2 of his RSA23, which starts breaking up at 1 or 2 on the volume. I suspect channel 2 of RSA23 is similar to the FTR37 that seems to have an unremarkable fender like preamp driving the AC30 phase inverter with cool biased 12AX7 and a big 47k tail.

I am going to build something to play around with these circuits. Interested in any words of wisdom before I dive in!

Hand drawn FTR37 schematic in this thread: https://el34world.com/Forum/index.php?topic=15506.0

1960 AC30 reference: https://www.thetubestore.com/lib/thetubestore/schematics/Vox/Vox-AC30-1960-Schematic.pdf

GA-20 reference: http://www.0rigami.com/gg/ga20t.pdf

 

I continue to read up on options ...

• debate on two 12AX7 stages rather than one pentode at the input
  
• claims that the pentode adds a "wide" or "thick" frequency response
• more tendency for microphonics with pentodes
• differences between EF86 and the 5879

Guess I am just going to have to build something and see what I end up with. My plan is going to be to provide a pretty standard 6V6 power amp section and separate the boards so that I can make significant changes. I plan to have two channels so I can AB the two. Probably will drill lots of holes for sockets, pots and switches so that I can make changes without having to make chips. If I end up with something I want to keep, I will make face plates. I will make this a head to be able to try different speakers and to isolate for microphonics if needed.