I just building the Apollo Mk2 from Aion which is a RAH clone too.
I guess you can finetune the lows with the first cap c1 3.3nF. This is indeed a bit like a treble booster. You can up this cap to 22nF or 47nF. If you use pin sockets, you are able to try several values.
Another thought if you like it tuneable just use the input section of the catalinbread naga viper (pedalpcb dragons breath) with two caps (3.3 and 68 nF)
I would not fool around with the tone stack, this is an exact copy of a vintage Hiwatt DR 103, it makes this signature sound,