Hi Thomas --- I looked at the ST70 Series II Schematic that Bob has placed in the schematics file.
The stock Series II has C1-C2 as 68MF @ 450V and C3-C4 as 100MF @ 400V.
Whoever owned the amplifier before you got it, apparently changed out C1-C4 to 150MF @ 450V caps or maybe a later Series II amp was produced with the 150MF caps.
On the original ST70 (circa 1965) the main cap was a 4-section 30MF-20MF-20MF-20MF can cap rated at 525V. This older functionally was like your C1-C4 caps, but all in one container(Quad Cap).
Over the years that can cap has been replaced by different 4-section can caps or by 4-individual or 2-individual caps. (40-40-20-20,80-40-30-20, etc). I have seen amps with larger capacitance in the C1 & C2 stages, like the 150MF caps that you have.
The intent of the large capacitance caps is to allow the amp to store more energy as it plays, thus giving you a smoother, larger sound.
At some point, how large a capacitance is overkill? The philosophy is that C1 & C2 should be large and C3 & C4 smaller. But what happens when you don't have the smaller caps available, well, I guess that you use 4 caps of the same size.
There has been some discussion on the Forum about the Can or Quad Cap replacement, so if you do a search, some info will turn up,
Hope that helps and does not confuse.