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

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. 

I didn't know the additional charge to build the Audio Note kit amplifier is only 10%. That's seems to me a very reasonable deal unless someone wants the satisfaction/challenge of doing the assembly themselves. 
Charles 

It is reasonable Charles, and if you look at their site, you'll see the beautiful job that Pete does. Besides that, the average guy doesn't have testing equipment. Also, if you want to use certain capacitors or resistors etc, they will accommodate that.

Also critical when planning for a low power amplifier is to know in advance what speakers you will want to drive with them.  It goes without saying that speaker efficiency and speaker impedance are of prime importance.

@roxy54,
Yes, the built quality looks excellent. That is why I was surprised the upcharge for this level of professional work is so modest.

@lewm, 
Absolutely right,  but I just assume that someone considering a low power SET would be aware of this proper speaker necessity. I sure could be wrong.
Charles