I would ditch the idea.
The transformer will not do anything to affect or color the sound.
In fact, you may be causing issues with the module.

As far as I can see on the schematic, the +16 & -16 Vdc is used for the opamp. IC 1.
C2 & C6 smoothing capacitors may be considered suspect as well as the ic.

Your + & - 16 volt supplies are being pulled down.

Awesome 'gift'.
Thank you sir.

I had to make a pdf of this.

Nice run down on the BBD circuit, Mandu.
thanks.

The amplifier had a failure.
Take it to a tech.
Attached schematic.

When I listened to the mpg file my first thought was "that's a bad speaker".
But then I saw that you verified them as good.
Glad that you got it sorted out.

The dc voltages don't look bad at all.
You don't have access to a scope do you?

Next step is to measure the output transistor voltages.
The first stage opamp (pin #1 & #7), the driver bases & the output transistor bases. (the left leg of the component)
No signal. Volts dc.

That is one heck of of a circuit description write up!!!
You have not noted what is wrong with the unit but a good tech should be able to narrow it down with that excellent info.

Where are you located ?
Are you wiring it for Domestic (110Vsc) or Export (220Vac)?

Here is the 'Domestic' layout.

Blk (CP8)and Wht (PS4) are the Primary leads from the Mains plug.
I can only surmise that the Wht wire (PS4) indicates Power Switch tab 4.

Red -Ylw- Red:  is the 45 Vac taps with Ylw being the Center Tap
CP1-CP2-CP3
Blu- Brwn- Blu is the low voltage Vac tap with Brwn being the Center Tap.
CP4-CP5-CP6

The tabs on the PCB should be clearly marked.

Please check the power supply voltages for stability, sameness & lack of ripple.
+ & - 42Vdc
+ & - 16Vdc

This may be the correct schematic:

Well you do have a working amp to compare voltage readings to.
First off, is R 165 cement resistor still good?