Any speaker can be flat on-axis if you use DSP/EQ.
The trick is achieving consistent directivity. With a 2 way speaker, a waveguide on the tweeter is ideal.
The best solution IMO is purchasing a high-end coaxial unit made by a company like KEF. You can buy the KEF R3 mid/tweeter unit for 113 euro.
Haut-parleurs, boomer, woofer,uniq, uni-q, medium, tweeter KEF, centre technique agrée par KEF depuis plus de 20 ans
setelec-shop.fr
Combine that with something like a Scanspeak 8535G00 8" woofer and you can have a really high performance 3 way speaker for under $500 each, including the cabinet + crossover. Do a hybrid approach, passive crossover for the mid/tweeter, active between the woofer and mid. That way you only need 2 channels of DSP and amplification per speaker. I would personally build them sealed and crossover to the subs at 50 or 60Hz. Ports can introduce some non-linearity, and if this is your first DIY project, just a headache to be avoided IMO.
A 2 way speaker can be done of course, but will have more distortion, particularly at high SPL. KEF Q350 drivers are also available for sale if you want this approach.
Haut-parleurs, boomer, uniq, Uni-q, woofer, medium, tweeter KEF. Centre technique agrée par KEF depuis plus de 20 ans.
setelec-shop.fr
Also, I assume you know this, but the acoustic treatment of your room matters a great deal. Getting a nice even decay rate across the entire spectrum is more difficult than selecting good speakers IMO.
