On the what sort of drivers are suitable for sealed boxes and which for ported, the conventional wisdom is if the Qts spec is below 0.4 it's supposed to be more of a driver for ported. Above 0.4 = sealed box. Around 0.4 give or take a bit and it's supposed to be OK for either.
But that's all general guideline stuff. And full of its and buts.
And I hate ports so much that if I were making a DIY speaker I'd start with sealed box regardless of the driver - and the frequency response and bass extension can go mate with itself.
The Eminence Beta 15 is a pleasantly affordable driver: £93 new from Thomann. I know that price often bears no resemblance to sound quality, but the 18" drivers that I've been looking out for, to try in my own DIY project cost ten times that much when new. I'm hoping to bag them used in good enough condition for under £150 each.
I did also explore getting 24" drivers, but haven't been able to find any that I fancy for reasonable money.
For sealed box enclosures it seems that the size that would look about right for the driver size gets you in the right sort of ballpark.
It also seems to make sense to make a cabinet that's a bit too big according to the online calculators, listen to it. Then add something to take up some internal space and listen to it again.
I think that there's also a lot to be said for having over-engineered cabinet sides, eg metal plated or over the top plywood walls. And for no internal wadding, or just a curtain at the halfway point to break up standing waves.
From my own personal limited anecdotal evidence, I think that the quality of the drivers is the most important factor in getting good sounding speakers. Especially in the midrange.
Edit: and something I forget to mention is that when it comes to speakers thare's more than one way to skin a cat. As in there's lots of different routes to getting good to great sounding speakers. Also with speakers there's a lot of compromises. Improve one area and you tend to get drawbacks in other areas.
I also think that there's a huge amount of value for money in DIY'd speakers. To the extent that just about anything that you make will give you better sound for the money than brand new retail off a dealers.... Although that is damning with faint praise. ![Very Happy :grin:](./images/smilies/icon/biggrin.gif)