Your profile gives no clues what country you are from. I know that in some country's there is a Right To Repair Law, but I don't know the details. Even if your country doesn't have such a law, you might be able to obtain service info from someone in a country that does.

Maybe just a power amp small enough to fit on a pedal board would be a good project. There are lots of preamps build into pedals so you are not stuck with just one preamp. If you need more or less power, you can just get, or build another power amp.

The original question in this thread was never answered. What amp to build ? I don't know if there is a solid state amp that people want to build. Maybe the Randall RG100 comes as close as anything, but I don't know that anyone has done it. There are a couple of threads about the Marshall Lead 12. Even some PCBs offered for sale.

Link: https://www.ssguitar.com/index.php?topic=5164.0

What you really don't find is a good layout. I mean a layout so you can build say a power amp  or preamp on some kind of proto-board. I used to use a PCB program to do layouts, but that software died so now I just do a 2X pencil sketch on graph paper.  For help drawing circles, I use a circle template. Attached is a scan of a layout for a power amp. I use three sizes of eyelets. Sometimes I use turrets to mount power resistors up off of the board.

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OK, I'm glad to hear that you have experience on tube amps so you know how to be safe.

Do you want to build a tube amp, or will you be building a solid state amp ? With tube amps, you can use techniques of old Fender or Marshall amps with eyelet or turret boards and chassis mount tube sockets. For a solid state amp, options are different. PCBs simplify things if you can find one for the circuit you want to build. I recommend you have at least three boards. A preamp, a power amp and a power supply board.

For a preamp, you can use the technique used to build a pedal. I like to use eyelets for any wires that go off the board and a one pad per hole type board for the rest of the circuitry. I hate vero board. I use a similar board for the power amp but use more eyelets for power transistors and large resistors and caps. For the power supply I mount almost everything on eyelets on either blank FR4 or a board with big holes on a large pattern. For most amps, you need to be able to drill and tap holes on the heat sink.

Using an IC power amp is usually a challenge to hook up on a proto board. It would be wise to look for a PCB.

The most difficult thing to deal with is the potentially DEADLY Mains wiring. What country are you in ? It's best to have a mentor that can look over your shoulder and keep you safe while you "learn the ropes".

Note the DC Voltages in the ovals on the Green channel. Try to get the same Voltages on the Red channel. Chances are the Drain Voltage is way too low. Increase the resistance of the resistor between the Source and ground, R22 or R26 to try to get the Drain Voltage into the right range.

There is no protection for input JFETs Q1 and Q5. To add protection, connect a SIGNAL diode like a 1N914, 1N4148 or 1N4152 between the Gates of Q1 and Q5 and the -12V source point D. Connect the end with the band (Cathode) to the Gate and the Anode to -12V.

Note: It's a good practice to always plug the guitar cable into the guitar (or effects board) first, then plug the cable into the amp. If you hear that loud BUZZZZ! out of the amp, you're doing it wrong. Turning the amp off or the Volume down won't protect the JFETs you can kill them even when the power is off.

"Cleaning the signal" is not something you're going to find in a textbook on electronic technology. They are likely used as general circuit components throughout the design.

Ceramic capacitors are low cost capacitors. When compared to the ideal capacitor, they are inferior at audio frequencies because they change capacitance with Voltage and temperature. Film type capacitors perform better in this area. Ceramic's good points are low cost, small size and very low impedance at high frequencies.

All these characteristics are taken into account by the Electronic Engineer who is designing the product and must weigh cost against performance of the product. Often, it is the Bean Counters in the back room that exert an overriding influence on design decisions.

104Z is likely a 0.1uF cap. That's 10 followed by four zeros pico-Farads.

Is the amp being plugged into a working, properly grounded outlet ? Try a different outlet. If that doesn't work, take it back to where you purchased it and see if it works there.

From your description, I would say the amp is acting normally. What would you have it do differently ?

The only difference between the diodes is the breakdown Voltage. In this amp, the diodes are not being operated anywhere close to the maximum rating so substitution is OK.

It would be a cool place to try one (and only one) of those bi-color LEDs that are one color with one polarity and a different color if you reverse polarity.

Does your amp switch both sides of the line like the schematic you linked ? If so, did you install the network on both sides ?

Is your cap(s) X2 rated ?

This effect is not what I would call "Diode hash". It's not any fault with the diodes. The circuit is doing exactly what the math says it will do. To get rid of the "1+" effect, use the inverting form of feedback. See attached. The one disadvantage is the low input impedance, so you really need an input buffer like the TS pedal.
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Where can I find out more about the EU's Right to Repair regulations ?