JLH 2003 Class A - how good is it?

Well, I can replace these cheap components with better transistors, capacitors and resistors, and the sound will probably get better from this.
I do not know anything about the heating of this amplifier, but I think that he needs massive radiators.

Buy a second-hand First Watt F5 clone by Tim Rawson. I have one! Forget that bag-o-parts Ch*nese kit! The John Linsey Hood is an ancient (1967) amp design!

Yes 1969 is an old 10+10w scheme. https://www.ebay.com/itm/2pcs-2-Two-Channel-JLH-1969-A-CLASS-A-Amplifier-Board-DIY-Kit-ST2N3055-MKP/...
But this is the new 2003 scheme.
First Watt F5 clone retails for $ 700. which is not comparable with the price of JLH.
Most likely for such little money you can’t buy a better-sounding amplifier.

JLH1969 https://youtu.be/bO5wRU8Of7Q

That Ch*nese kit looks like junk! Where's the power transformer? What about. the heat sinks? In this life you get what you pay for!