The thing about diy'ing audio (primarily speakers but also amps, DACs etc) is that you can get top of the line performance for a fraction of the market price. A $50,000 speaker setup that would bring tears to your eyes could be made for perhaps $5000. A DIY $500 kit can perform similar to a $2-3000 set of speakers. Open source amps with gerber files on github are amazing.
The biggest reason it's so easy to get amazing value is because that $600 speaker only has $150 of materials. Upgrading its $25 woofer to a $80 one would help a lot, but no company would do that and not sell it now for $1000 if they could.
However the biggest allure for me is not beating commercial systems on cost, but making what I want. A small speaker with deep base? Easy. Speakers with quasi-active noise cancellation behind them? Sure, why not. Speakers that'll make the most overpowered/fancy beach-boombox sound like a crappy toy? Simple.
The only limit is your imagination and time/money.
I'd very much recommend diyaudio.com, but be warned, parts of this field are mature while others are still in effective infancy. Also, being an engineer (electrical/mechanical) helps a lot, there's a ton of signals processing and electrical/mech oscillation.