6500 RMB is a normal price for an used Marshall 1959slp if it is not a HW version.
If you wanna install a master volume, go get PPIMV LAR/MAR version 2.
You only pay 50 US dollars including bench fees and components to get your amp useable in the bedroom.
I have a 1987 MKII which is a 50 watt version of 1959slp, and I put a PPIMV LAR/MAR ver2 in the rear.
It works very well!!! I can even play along with it in the middle night without any complaints from my roommates.(God bless them)
Although it does change the sound/tone a bit, that's the best solution(at least one of the best) that I can find at such a super low cost.
The second best way is to install a London power scaling which costs around 200 US dollars.
It reduces the power output to 0.1 watt or even less to get a great sound/tone.