If you're looking for a pro-audio subwoofer -- one that won't run out of linear Xmax at full power down to (say) 30Hz or below -- there are few choices.

Taking linear Xmax as (Hvc-Hg) and mathematical Xmax as Hvc-3Hg/4 (estimated to agree with DUMAX or Klippel Xmax), we have in order of descending mathematical Xmax (which is what most manufacturers quote):

1. Aura 1808/Seismic 8196 : 100mm underhung 25mm Al coil in 50mm gap

-- linear Xmax 13mm, mathematical Xmax 19mm

-- replaced the 6174 (see below), has lasted me for several years now

-- extremely expensive

2. McCauley 6174 : 100mm overhung 40mm Al coil in 10mm gap

-- linear Xmax 15mm, mathematical Xmax 17.5mm

-- relatively low BL, I fried one of these with repetitive 25Hz bass

-- very expensive, needs huge box

3. BMS 18N850V2 : 100mm overhung 38mm Cu coil in 12mm gap

-- linear Xmax 13mm, mathematical Xmax 16mm

-- just plain expensive, I will use this next time (since I need 4 drivers)

4. Ciare 12.00SW / 15.00SW : 100mm overhung 33mm Cu coil in 10mm gap

-- linear Xmax 11.5mm, mathematical Xmax 14mm

-- not much margin past Xmax before damage?

5. B&C 21SW150: 150mm overhung 32mm Cu coil in 12mm gap

-- linear Xmax 10mm, mathematical Xmax 13mm (B&C quote 15mm...)

-- extremely expensive 21" driver

6. B&C 18SW115 : 115mm overhung 29mm Cu coil in 11mm gap

-- linear Xmax 9mm, mathematical Xmax 12mm (B&C quote 14mm...)

7. Ciare NDH 15-4S : 100mm overhung 30mm Cu coil in 14mm gap

-- linear Xmax 8mm, mathematical Xmax 11.5mm

8. PD 2150 : 150mm overhung 30mm Cu coil in 15mm gap

-- linear Xmax 7.5mm, mathematical Xmax 11.5mm (PD quote 12mm)

Below these there are a lot of drivers with 25mm or shorter coils which shouldn't really call themselves *sub*woofers...

Out of all these the Aura/Seismic driver is probably the most linear, especially because there's very little flux or inductance modulation due to the underhung radial neo magnet structure. However it's not suitable for many enclosure types due to very low Qes, and is *very* expensive.

The McCauley needs a huge box and is (in my experience) a bit fragile, and it costs almost as much as the Aura (if you can find one).

The BMS is probably the best choice overall, it's pretty linear and quiet at high amplitudes, and works well in relatively small reflex boxes e.g. 140l tuned to 32Hz is close to ideal. It's about half the price of the Aura/Seismic driver.

The Ciare drivers are good and quite cheap but don't have as much displacement because the diameter is smaller.

The B&C and PD 21" look good at first, until you realise that they cost more than a pair of BMS18N850 and have lower power handling, efficiency and displacement.

Overall I'd say the BMS18N850V2 is the best bet -- the V1 has higher BL and Mmd which again makes it less useful except in a very big horn.

Ian