One of the reasons that you're having a problem with gain with those preamps is the change over the years in head design. The early heads, particularly full track mono, were big. So big in fact that when they first introduced 2 track mono and then 2 track stereo, the heads wouldn't fit side by side and still place the gaps in the right place. If you've ever noticed, the early 2 track tapes were specified as "in line". That was opposed to staggered heads where two heads played back the tracks from a different location (staggered) on the tape. You can imagine the problems encountered at that time with each company championing a different standard and today, archivists either have to adjust the location of two heads or record each track digitally and "drag" one track to line up with the other.
Oops,.. that was a little tangent but back to the gain issue, one of the reasons for the size of these heads was to get as much output as possible to meet the demands of the tube electronics of that era.
So, the head needs to meet the needs of the electronics or vise versa.
In general, mating these early repro amps with a more modern head doesn't work very well. I think that even if you were to set up the ideal ratio of input and output impedances, you still wouldn't have very good performance.
It is possible to have heads installed that are more "tube friendly" but that's a little far to go to see if those amps have potential.