Because of what ?
1. the slot,
The slot will cause a spike in spl at a frequency related to it's dimensions. The spike may not be as bad as a sim predicts, but that's what the slot does. This can be anywhere between benign or problematic. The only way the slot could possibly sound better than slotless is if you actually need that spike, high inductance drivers will have a falling response so the spike could be beneficial to keep response flat as frequency increases.
2. the cancellation of even order harmonics,
The cancellation of even order distortion is the only audible benefit and it may or may not be worth the trouble - more on that later.
3. the cancellation the mechanical vibrations
The mechanical vibrations we are referring to here don't have a sound. The pressures in the box remain the same so what you are cancelling is the mechanical force of heavy cones moving. This can stop the box from physically rocking or "walking" but it won't sound different apart from the even order harmonics cancellation (except for the imperceptible loss of spl from the forces lost to rocking the box).
There are probably 3 main factors that will determine if you prefer the cancellation of even order harmonics.
1. How much even order distortion is there in the first place. This type of driver loading cancels out distortion due to non linear stroke in the forward stroke direction vs the backward stroke direction. This type of distortion can be due to the suspension, the Bl or the inductance.
Some drivers made by manufacturers that use a Klippel during the driver design phase are very linear and this type of driver won't benefit nearly as much from stroke non linearity cancellation as a cheap garbage driver with loads of even order distortion.
This type of linear driver is typically very expensive but measures very well on a Klippel and is very symmetrical throughout it's stroke. See the B&C 18TBW100 Klippel results as an example -
http://circuitcellar.com/wp-content...kasonTBench_BCSpeakerReprintedwPermission.pdf
2. Whether or not you like the lean and clean sound of low even order distortion. This is not a given, in fact the fuller and very likely psychoacoustically louder sound afforded by 2nd order distortion may actually be preferable.
3. How much odd order distortion is present. If you take away all the pleasant sounding even order distortion and leave large amounts of odd order distortion the sound can be quite bad. If the subwoofer has large amounts of odd order distortion of course.
We all choose our favorite compromises. DJK very famously likes PPSL, whereas PPSL (and Servo for that matter) are things I would never even consider. PPSL requires a good amount of box volume for the loading chamber (compared to slotless) and Servo is expensive. And the only benefit to either is cancellation of even order distortion.
On the other hand there are two other approaches that are far more to my liking. Doubling up subs (both drivers and enclosures) and/or horn loading provide the benefits of lowering even order distortion (for a given driver, tuning and spl level) as well as lowering odd order distortion as well as a bunch of other benefits including the ability to get a lot louder. These two options are quite a bit larger and more expensive than the PPSL or servo options but they also have a lot more benefits.