Sextett =  box with passive radiators. Effectively monopole / closed-back. The "sealed" term above includes "ported" cup designs. (T50RP mods with open ports, Denon D2/5/7000 [yes, they are ported], Darth Beyers, etc.)

...

FUN FACTS:

• Dipole radiators don't need a seal or as strong a seal to maintain bass extension. In fact, most or not all dipole radiators types tend to have some bass roll-off, e.g. Grado, HD600/650, AD2000, etc.
• Monopole radiators like most orthos, stats, and some closed cup headphones will require a good seal to get good bass extension.

I came to understand this after taking measurements with simulated open air coupler (foam) and solid-plate couplers. I had always wondered why "open" (or more precisely open-backed) headphones like orthos acted more like closed-cup monopole headphones during the measurement process.

Thanks, whenever I have to I can now explain what an AKG Sextett headphone is. This has riddled me for years even though I loved the sound of the Philips/AKG N6330 Sextett.