Indeed, room correction is just one aspect of DSP, and it's one that I'm having less confidence in all the time, after experimenting with it quite a bit.
A study was done on how much DSP could improve room clarity, and the answer was basically that it can't, at least not in a 2 channel system. A good room doesn't get any better with DSP, and a bad room usually doesn't either, unless you are sitting close enough to the speakers, where room problems tend to be simpler and lower in magnitude. My experience has been the same. Room correction can't do much about reverberation.
One thing this same study showed is that a good speaker does help! It makes sense that the room can't be any clearer than the speaker, so a bad speaker starts you off bad, and the room can only make it worse from there. And this is where DSP is interesting because using DSP to create crossover networks can allow excellent frequency and phase response with minimal comb filtering from driver overlap that is simply impossible without it. And, my experience shows that when I get my three way speakers dialed in with good crossover settings that give me a nearly perfect step response, the room clarity at the listening position measures significantly higher. It's true that you can get a good minimum phase step response with passive filters, but it requires 1st order crossovers which bring a lot of headaches and sonic issues of their own. With DSP you can get excellent minimum phase or even linear phase response with minimum overlap crossovers. I'm not sure it matters taking it all the way to linear phase, but I can do it with the flip of a switch, comparing uncorrected phase to corrected. It's a subtle difference, but after a lot of listening I tend to leave the phase correction on.
I know a lot of people feel that digital does something to the sound that's bad, and I'm not going to doubt them. But it also does things to the sound that are very, very good if used right, and those things can't be done analog. So the issue comes down to what matters to you.
As for DIY
, I would never consider spending a lot of money on anything other than DIY. If you really think about it, there are only levels of DIY. It's not clearly defined. Very few people make their own electronic components or speaker drivers. The furthest you could get from DIY would be to have someone else pick everything for you, including the listening space and all furnishings and treatments.