Is that the first time you pick up the 50Hz signal ?
What about on the input jacks, both sides of the 10k resistors and also both sides of the 100nF capacitor.
Also check both sides of that 470k resistor to Ch2, I like to measure on the components leg if possible since the signal could be picked up in a component or a track.

If you can't isolate it that way then probe with a wire to 0V ( ground ) on these areas and see what happens with the signal.

Components sometimes work like a great antenna for the magnetic field around that transformer.

I would like to add that some heat transfer paste is conductive and will cause shorts if used with heatsinks and mica washers.
The conductive heat transfer paste is often used in computers for the main processor heatsink, it makes no difference in there since there are no electrical terminals close by.

Make sure you only get non conductive heatsink paste if you get some, a small tube of really good expensive paste will last forever and ever so don't get anything cheap and nasty.

That is the wrong type of jack to use in that amp.
The ring should not be connected to the chassis, it is not at 0V.
You need a jack with a plastic insulated mounting ring that does not connect the ring to the chassis.

The KiCad schematic was supposedly traced out by someone from their amp, I would trust the other 2 more as well.
If you cant see the schematics in full scale then right click on the small picture and open in a new tab or window and it should come out larger.

Try these

Love it, its going into my to do list, I might even make a tiny PCB up for it.
I might even make one in a stompbox and use it as an effect pedal.

PBTL is just 2 amplifiers working opposite to each other, one puts out a positive signal while the other one a negative and then it reverses during each sinewave.

Usually the speaker negative is connected to ground and the positive to the sinewave, the ground is say 0V and the other +-12V so you get a swing of 12v on the speaker coil.

In PBTL when the positive on the speaker gets +12V the speaker negative gets -12V, and then the reverse in each sine wave, thats a full 24V swing across the speaker coil causing more movement and power.

The load on a PBTL is worked out different then just a single amp module because both amp modules share the load between them.

Where did you get the cassette adaptor ?
Whats the brand and model of it ?
A nice picture speaks a thousand words.
I really like to try this as I have some old cassette players somewhere.

If I have to remove any components I usually cut all the legs off first and then I can remove them one by one.
Components are cheap and unless I have to remove an expensive part just for a test the legs get snipped.

Why do I always get a cold shiver when I hear or say snipped?
It must be a guy thing I guess.

I just cheat and find a calculator on the net.
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php
http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php
http://sim.okawa-denshi.jp/en/Fkeisan.htm

Sometimes these tiny amps surprise you when you hook them up to a good speaker, many people think that they cant drive a 4 x 12" cab but "surprise"

If you are talking about the capacitor between the FET and the volume pot then that might be your problem.
When you use a capacitor and a resistor together you end up with a filter, in this case it is a High Pass filter that only lets frequency's above the set filter frequency through.
The 47nF cap and the 10k pot makes a high pass filter at 339Hz
The 22nF cap and the 10k pot makes a high pass filter at 724Hz
Nothing below those frequencies will be heard, you eliminate them.
If I ever made that amp I would use at least 220nF to allow bass frequencies through, but that is just me, it is set a such a silly high HP filter setting to play through a crap speaker.

Have a play with this calculator to see what I mean:
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php#answer1

The efficiency of speakers are not talked about that much for guitars but when you get to bass guitars it often comes up.
A good efficient speaker and cab can double the loudness of your amp.

My first tube "anything" did not have a standby switch, but it took about 5 minutes before any noise came out of my 1957 VW Beetles all tube radio.
It freaked some people out how the radio came on all by itself, it just needed time to warm up.

I think that PCB is not well designed, you will need a good heatsink on the power transistors but the transistors are right in the middle of the PCB, right behind the transistors where you should mount the heatsink bracket are 2 small footprints for 7 watt resistors.
Have I been asleep for centuries and 7 watt resistors have now shrunk right down in size ?
I would throw that PCB in the bin, I would never try to build that amp since it will undoubtedly lead to heartache.
You can buy a 100 watt class D amp for less then the cost of the components you need to complete it, and the transformer will be expensive as well. There are so many better and cheaper options out there.