Say a fella wants to build an SET monoblock based on the 300B..

The kits are one way of learning but you also get what you pay for.  Why use that tube?  What are you intending to drive?  If you are looking for a beautiful sounding tube use the 45 tube IMO.  If you are going to build an amp go Class A design.

Happy Listening.

Check out the 300B set amps on ElliotAudio.com. He’s got several designs suitable for your use. 

The Elekit 300B is wonderful.  But, it's not mono block.  

The Audio Note amplifier kit is granted more expensive than the other good 300b kits mentioned above. In my opinion it’s money well spent.
1 Very sturdy and high quality aluminum chassis.
2 Very good quality standard parts. High quality power supply.
3 The option to upgrade capacitors and resistors and especially the output transformers (C-cores are available)
4Tube rectified circuit
5 Proven track record x 25 years.
6 The potential to build an upper tier high end quality 300b SET.
7 300b offers more flexibility in matching speakers due to higher power compared to the 45 and 2A3 tube options.
Charles

I agree with Charles. I have had 2 of their aamps built by their sanctioned builder, Pete Fulton, whose build quality is immaculate. I also had him build one of their tube DACs. In each case I was very satisfied and they were totally reliable. 
You Might be able to do it yourself, but for th additional 10% charge, you have the peace of mind of knowing it was done right, and the resale value will be better as well.