Nice videos, but I'll point out that the two input jacks are not high and low gain. The high input is a low cut filtered input making it a bright input and the low is a normal input.
The tantalum caps and the green 1% film resistors that were used in these amps were state of the art at the time these amps were designed. They were expensive to use, were far more reliable and helped keep some of the typical noise levels down compared to other amps being built at the time.