I also successfully completed my modification to the bass frequency. Figuring out the schematics took a while especially since it had a few famous Sansui typos.
On the 8080 tone control board F2544 instead of a frequency selector switch and several capacitors the 8080 has these 4 capacitors, they are the same capacitors as the "150HZ" option in the 9090 at 0.033uf, they are C20, C21, C23 and C24.
The schematics for the switch position for S01a is a misprint, it should have the numbers in the reverse order as 3, 2, 1.
In a 9090 0.012uf capacitors alter the bass frequencies at 300HZ and 0.033uf capacitors alter the bass frequencies at 150HZ. I don't think this is entirely correct as my AU719 bass controls at 150HZ sound much deeper.
What I did was solder some 0.012uf capacitors in parallel with the existing 0.033uf capacitors in place. I used MuRata MLCC C0G 50volts 0.012uF capacitors. Part number: RCE5C1H123J1A2H03B
https://au.mouser.com/ProductDetail/81-RCE5C1H123J1A2H3B
I did this on the reverse side of the board as there was more room and it is easily removable if needed in the future.
So far I'm very happy with this mod. With them installed it sounds exactly the same as my AU719 does when in 150HZ mode. For me this really completed the amplifier, it removes the high bass boom and replaces it with a deep curve boosting only the lowest frequencies needed on my Monitor Audio RX1 bookshelf speakers.