Yup. Well written, thoughtful, and an nice dry sense of humour. Self caused a bit of a quiet revolution in amplifier design. You can see his influence in many many commercial and hobbyist designs. His “blameless” design remains something of a benchmark. You can get PCBs for his designs, and full kits, although they are a little pricey. For a medium to high power domestic power amp these would be very hard to beat. The Compact Blameless or Trimodal run as Class A might well suit the OP perfectly.
The audiophile arena was always the strongest, apparently because high import duties made it financially sensible to build your own gear. I’m not surprised that tradition remains. I think all that’s left in the US is the tube crowd.
Most general electronics can be bought so cheaply that there’s hardly even hobbyist motivation to build any. What is built these days seems to take the “use a field-programmable microprocessor and dumb it down” approach rather than the “build it up from functional parts” approach of yesteryear. Not denigrating, I just never found that approach interesting at the hobby level.
I still have a full bench and occasionally whip up little doo-dads that are cheaper or far more task-specific than I can buy, and even more often buy gear with the intention of modifying it to suit me.
There are many clues, but I’m not going to give any here. 
I was well-acquainted with Forrest Mims, Brian Fenton and… oh, damn, who was it that edited Radio-Electronics for decades? I also wasted far too much time working on proposed projects for Radio Shack (in the Mims/hobbyist days) that never went anywhere because Tandy was a perpetually HUA company. ETA: My audio-gear designs were few and minimal. I worked mostly in tinkering, digital gear, household convenience and the early years of satellite and digital TV signals like teletext.
Ah, the days. (Gazes at the faded scar on his thumb web from a particularly nasty flipped blob of solder…)
if you want to tinker, buy and amp and build some speakers.
people put lots of effort into speaker enclosures and room acoustics using voice coil dynamic speakers.
or build some electrostatic speakers. these make a huge difference if you make them well.
I still dont have speakers. I want to build a pair - 150 watts, maybe? With a subwoofer. I have access to 120 and 250 volts. I dont want something bulky and I want to build it myself - no used stuff. I want to have something unique. I saw once on a speaker project a part that separates the sound in two - for the woofer and the tweeter. Can I implement this part on my project so i can adjust the sound? What about balance? Are car speakers ok for this project?
Building good speakers is WAY harder than building an amplifier.
The old rule of thumb is “put your money in your transducers.” In other words, it’s easy to build a cheap Amp that sounds OK, but it’s really, really hard to build cheap speakers that don’t sound like crap.
I once built my own subwoofer. It took several months of research and planning, and some test equipment (signal generator, Amp, and oscilloscope) to get a design that sounded OK. I’m sure I could have bought one that was better for less money.
But, it was fun, so if that’s the reason you want to do it, go for it.
But, don’t expect to be able to save money doing it yourself.
Antique Electronic Supply is your friend. They have all sorts of new production and old stock tubes, chassis, transformers, etc. The old TV and Radio magazines from the 1950s had a ton of DIY tube amp projects, some of them with tubes that are still in current production. The hard part is the variety of transformers is a lot more limited.
Ah, OK, you are really starting from a very low base here. Lots of questions that say to me that you need to slow down a little, and learn about the area more, before deciding on a specific idea. Experience (hard won) has shown that it is all too easy to get fixated on one part of the idea, or to start on something too big in the beginning, and never finish it.
You can do really well building your own system, and gain a lot of pleasure from doing so. Building speakers is great thing to do as well. But it sounds like you really need to read up on the technical aspects before doing anything. And before you go too far, you really need to set a budget. You can spend any amount of money here. I can point you at speaker kits that are really good that cost $50, or kits that cost $2000. And that is just for the drivers and ancillary components. Something like a Zaph Audio 2 way kit is phenomenal value. But add on the cost of making boxes, plus your time.
So, start by working out what you really want to get out of this, how much time you have to spend, and how much money you want to spend.
If you have to ask if you can use a crossover, you’re way over your head, I fear. You may want to read up on things like Butterworth and Linkwitz-Riley designs, but if you want to be able to adjust the sound, an active crossover may be what you need. If you go this route, you may as well use parametric EQ feeding monoblock amps, where each driver has its own dedicated amp. This can get really complicated and really expensive really fast.
Car speakers are probably not a good choice as they tend to be 4 ohm rather than 8, so you can get into trouble with the impedance and overloading your amp.