Hackworth's customer's video is a great example of a principle often mistunderstood by the tube amplifier novice. Tube amps sound very different at the different volume settings. To get the rich, warm, and sometimes fuzzy sound from these amps you have to get the dial up higher. This is due to the vacuum tubes themselves, they are very non-linear, and produce more of there distortion at the higher end. If you run out and build a 40watt amp and later find you have to keep it turned down low all the time you will displeased with the sound. Unless of course your desing has some type of master volume circuit that allows you to run the tubes high, but shunt some of the output off before the speaker so your output remains low, thus allowing you to run in the distrotion risch zone at low volumes.(important for late night practice playing when others are asleep.)
Second point, Watts has alway been mistaken for loudness. It is not a direct conversion. the biggest variable is the speaker effieciency. Years of old the speakers had sensitivity in the high 80's to low 90s of decibels (db) per watt at 1Meter. However now there aree much higher efficiency speakers, mainly due to advances in rare eareth magnets such a neodidium, wher you can buy a speaker that has sensitivity exceeding 100!!!! So you say 100 is not much diff tha 90, but it is not a linear scale. Basically every 3db is like you have doubled the power!!!!
So if you build one of hackworths 5 watters and put a really high sensitivity speaker on it you will have both loud and you will be able to operate in the "warm and fuzzy" part of the tube's curve. Combination to me is pure heaven when playing the blues, as was so clearly demonstrated in the video.
My suggestion: Start by reading a book, there are several very entry level books such as Dave Hunter's that are written for the layperson as opposed to an engineer and will really shed light on all the areas. You will be much better informed in making your decision, plus you will have some understanding about what you are building and how it really works. Then buy a kit of parts from somebody reputable and caring such as Hackworth. You dont have to buy chassis and cabinet if you dont want, or even the board. You will find kit suppliers give a lot of choices as to what components you want as well. You will not beat their prices by buying the parts onesy twosey yourself from big companies such as grainger or digikey, your volume is just too low.
Last tidbit: Guitar is all about distortion. If you had a really good audio amplifier with lets say less than 1% total Harmonic distortion (THD)and played your guitar thru it you would hate the sound. 10% THD is considered a clean sound in guitar, a "dirty" sound being much higher. That why tube amps have remained alive and well, they are great distortion boxes, with theri distortion being smooth and warm, pleasing to the ears. Solid state designs have to try and artificially, so to speak, add this distortion back in to emulate the tube sound.
Welcome and good luck, as an older electrical engineer I like to see young people want to build something as opposed to buy it. You will learn alot and cherish it the rest of your life.