Why do planars have poor bass extension?

"Mapman, I suppose your key phrase here is "good cone bass"! How many such speaker systems exist today? "

I suppose what is considered good is subjective, but I would say many if set up well and driven well with the right amp.

"you mean it's easier to match an amplifier to a cone driver speaker than a planar speaker?"

I'd say smaller, less powerful, and accordingly less expensive amps in any particular line might be found to fit the bill better in general with many box designs compared to planars. At least that is what I have experienced.

Newer high performance amp technologies like Class D switching amps where you essentially get a bigger high performance amp in a smaller package might help render that distinction as more of a moot point though I suppose.

Good bass extension can be achieved with adequate power.

If the bass area is large enough they can have a lot of bass. My Apogee Duetta Signatures have very impressive bass and the bigger Apogees had even more.

You need to hear some Apogees driven well. No bass problems there...

You can have low bass with proper room placement, no piles of equipment and racks, behind, and between the speakers.

Also an adequate sized, but not large room, with not a lot of furniture, and sit in the right spot.

Read "Get Better Sound" by Jim Smith for great info.

My Magnepan 3.6's go down to about 30Hz in the several different rooms I have had them in!

It takes time and careful listening, if you take the effort.