Hello from Colorado.
I have been building speakers mostly DIY since 1969. The “art” has changed a lot over the years especially since Thiele/Small modeling in 1973 was published.
One HT sub-woofer I made in the last couple of years involves a Stereo Integrity HS-24 in a 14cuft box. Flat in room response to at least 10 Hz.
I have built also electronics from parts but now mostly focus on loudspeakers