Since discovering that the Beyma and RCF lf motors could be purchased at "nice" prices I've been plugging lot's of them to see what of interest if anything I'd come up with. The TAD drivers are of great interest to me as well, but I'm less interested in their pricing!
Beyma makes one driver that's of particular interest as it's specs are quite similar to the TAD TL-1801, which is the TL-1602's big brother.

TAD 1801
Vad 17.7
Qd 2.7
Fd 26 hz

Enclosure 6.56 ft3
Ideal Fr 27.34
Ideal Q 40.5
Encl. Fr 27.34
Encl. Q 6.34

Beyma 18LX60
Vad 17.67
Qd 2.7
Fd 27 hz

Enclosure 6.5 ft3
Ideal Fr 27.34
Ideal Q 40.5
Encl. Fr 27.37
Encl. Q 6.1

When I plotted the recommended PiAlign enclosures I got the following data:

TAD TL-1801
F3 40 hz
F10 25.8 hz

Beyma 18LX60
F3 42 hz
F10 26.5 hz

By comparison the TL-1602 offered 42.7 F3 and 31.4 F10. The 18LX60 is the third woofer down in the Beyma line. The other's may offer better overall performance with less extension.(speculation) I highlighted the 18LX60 because of it's very similar performance to the TAD's. It looks like an affordable alternative for someone wanting to get well into the 20 hz. range. Any opinions?