Understanding your speakers will help when you try to tune subwoofers, whatever the brand, to aid the bass.
The Soundlabs, as well as any other stat or dipole that runs down into the bass, will exhibit reinforcement at some frequencies (constructive interference), cancellation at other frequencies (destructive interference), phase shift (various degrees at all lower frequencies), and there is absolutely no way around those realities of physics. Folks are prone to saying "but oh, you have to hear my system, I have no problems" but that is simply wishful thinking. As long as folks will tend to define themselves by their possessions the "mine is perfect" syndrome will rear its ugly head. But the compromises, fairly big ones, are there at the lower frequencies.
Most folks don't want to use a high pass filter on such a large speaker that is capable of low frequency output(and this is the fatal trap, the ability of putting out low frequency energy, lulling folks into wanting to use it, but this is different than putting out relatively flat, somewhat low distortion low frequencies, which no full range stat can do) but blending will be more difficult if one lets the Soundlab run down low. No matter what the sub, it cannot track the uneven bass of a stat and fill in where needed. Most folks have blamed subs for being "slow" when problems arise when blending a sub, but in reality even a perfect sub will make the problem worse when it is overlapping in frequency coverage with the stats.
By the way, there is no "slow" sub and no "fast" sub. The erroneous terms "fast" and "slow" have arisen when folks have tried to describe the sound of subs that have low ("fast" sounding) or high ("slow sounding) harmonic distortion, or when a low distortion sub overlaps the frequency range of a monitor speaker, creating excess bass at certain freguencies.
If you do not want to put a high pass crossover in front of the Soundlabs you do have an uphill battle. That is not to say that you cannot achieve better results than you have now, but there will be no "MAGIC BULLET". Your best bet will be to obtain the lowest distortion sub you can that also has a variable crossover point as well as the ability to change the low pass slope.
The JL models previously mentioned are an example of good subs that have good adjustability. But they do not come with a high pass crossover for your speakers, should you want to try that route. They do have a new crossover that is sold separately but I cannot speak first hand of the quality.
Remember, it is not possible to beat physics, and if it were there would have been plenty of full range panel speakers with great bass over the years. The Martin Logan CLS, the Apogees, the Soundlabs, and we could go on and on, all had their problems in the bass. The only stats that don't have real, apparent non-linearities in the bass are those that have little bass to begin with so the problems are not as easily noticed. Those are also the ones that are easiest to blend subs with....and it sound here like I am advocating a simple high pass filter on Soundlabs. I am, despite the seeming paradox of having a big speaker and not using the bass from it.
