[Lincoln]

Hi,

I'm very out-of-date, but I just started to build this amp. Did you installed brass plated? Also, one thing I don't understand, I'm an amateur on building this amps. What's the point of using a brass plate or bus ground if chassis already connects pots?

Welcome. The brass plate gives a better ground connection for all the pots & jacks. It also serves as a ground bus of sorts - the board componants are grounded to it also. And it is easier to solder to than the chassis is. Soldering to the chassis is hard. Can't get it hot enough in a localized spot for a good connection.
If you have a brass plate, I would recommend you use it.