I had a chat with Dom about this. Dom ran Pure Filth for a very long time, and built the majority of the soundsystem himself. He also builds speakers for other people. He's very clued up on this sort of thing.

His general advice was that if you're building PA then it's worth building the speakers yourself as you can achieve similar results to the manufacturers. Soundsystems are rarely mass produced and if you're prepared to do a bit of learning you can save yourself some coinage by doing it yourself.

However, if you're building small speakers for home / studio listening then you're unlikely to beat the manufacturers. Professionally built Hi-Fi speakers and monitors have a lot of high quality R&D behind them which you can't possibly hope to emulate. Plus you won't have the economies of scale that a manufacturer has.

[Waits with baited breath for Dom to show up and say that I'm talking out of my arse]