They aren't even close. They will not work without a major redesign of the circuit. They might work a little better in the stock circuit if the LEDs were replaced with resistors, but you'd be moving backwards to use an inferior tube.

The 6C8G is not a substitute for a 6F8G/12AU7.

The Speedball will increase current delivery to the load at the output, which will increase available power. The stock circuit has a ton of voltage swing; that's rarely an issue.

Most IEC power cords will be stranded copper with a PVC jacket.

Solid core copper. If it appears to be silver, it is tinned to prevent the copper from oxidation, which is not at all abnormal and will allow the wire to be reworked easily at a later point. Stranded wire should not be used for these purposes.

Encouragement would be saying something like "go ahead and do it". The statement that there are many mistakes in the clones is some evidence that there isn't approval on that end. I don't see any encouragement in anything you've posted from that statement. The statement also talks about...

I don't see any words of encouragement there, that seems to be your addition to the conversation.

I sure hope the winding isn't 15VA, the Crack amp draws more than 30VA from that winding; the SEX amp draws more. AC and DC current are not the same. The transformer only cares about AC current.

The Crack transformer is also in the SEX amp, and that's the clue to the capacity of the HV winding. From the original SEX amp and Crack 1.0, that indicates a high voltage winding capacity of 72VA and a LV winding capacity of 22VA, so I guess it's only 94VA. This is to keep the transformer...

The stock transformer is spec'd at about 105VA and runs under 50VA.

For those who may consider a larger/cooler running transformer to be some kind of improvement, it's worth remembering that underloading a transformer leads to poor regulation, and poor regulation can leave your 6.3V heater winding putting out far more than 6.3V, which can reduce tube lifetime...

The large Speedball board heatsinks dissipate the same amount of heat as the 3K resistors do, but both the large Speedball board and the small Speedball board have some additional current flowing through them to permit the current sources to operate, so the overall heat dissipated inside the...

The heat dissipated under the chassis goes up when the Speedball is installed. The localized heating of the chassis plate right where those 10W resistors sit will decrease when the Speedball is installed. (Always remember to install the rubber feet too)

If your amp passed the voltage checks, then it is running as hot as any other working Crack. The kit has been out for over a decade with no reports of failing wood finished from heat, and consequently I would suggesting using whatever you like.

No amount of parts swapping on a stock Crack will bring it up to the performance level of the Crack-two-a. Granted there's a lot of enjoyment in parts swapping.