There's a world of support for builders at DIYAudio Forum and Planet 10 Hi Fi.
Another interesting area is the Gain Clone or Chipamp amplifiers: use opamps like LM3886 and LM1875 to create really good sound.
Now look at the speaker in this old picture of Johhny Cash: That's a classic Karlson horn loudspeaker.
Reckon my Hi Fi needs some work though, so I've been looking to replace the 8" bass unit (185mm cutout) in my Monitor Audio R300 infinite baffle speakers with a full range Fostex unit, and replacing the tweeters with a bass reflex tube since the ferrofluid in them has dried up and they sound harsh.
Looks like a whole heap of fun to me, and no crossover required.
FWIW, the smaller Fostex units have a huge fan-base too. I have some old Marsland projectors that I've upgraded but still retain their old look.
There was a good example of Metallica's recent album, where there were uncompressed tracks released with one of the guitar hero games and you could compare.
You can also simply replace the 8" bass unit in bass reflex speakers with the sturdier full range Fostex FF225WK.
This is a high end example, but people also just join a few resistors to these chips and connect a power supply in real cheapie designs. Interesting to read The Death of High Fidelity, which explains just how Hi Fi is regressing with horribly compressed MP3 technology.
I hand tuned and ported both of my three-way speakers.
The first three numbers will be 220 which denotes the manufacturer as Jensen.
The next number is the year of manufacture and the last two numbers will be the week of manufacture.