Late to the subwoofer party...
It seems the goal of adding subwoofers is to augment the main speakers in a manner the that is wholly indistinguishable from the mains. The trick is how to do this. There are a lot of pitfalls. I am not saying I am an expert in this field but I have experimented with several different subwoofer brands, the number of subwoofers in my room and types of integration. So I will offer some of my thoughts and experiences.
In my limited experience I have observed that not all subwoofers are created equal and as one of the posters above pointed out many of them are aimed squarely at the home theater market. The issue of getting a pair of great subwoofers that are going to perfectly integrate leads us to the main issue -- money. What we typically see is that someone has a pair of $100K+ speakers and wants to buy a $3k-$5k (or maybe less)subwoofer that has built in, off the shelf, plate amps and use cheap cables to connect it. This is quite a disparity. It is no wonder the result is often not satisfactory and the owner of the subs throws up their hands and just says "Subwoofers just don't work with my speakers!". As cmarin quoted his setup guru as saying, paraphrase: "this is going to take a lot of time and money if you want to get it right". If we want the great, deep bass, along with the other goodies that come along with it like soundstage and envelopment, then we have to start treating the bottom octave with some respect.
And once we have the perfect subwoofer there is the issue with proper setup. Even a perfect subwoofer not setup well is going to sound at best mediocre. Getting this part right is extremely challenging and requires a very good ear and a lot of experience. Just my opinion but If someone is trying to integrate a pair of subs with measurements only then it is not going to end well. I posted in another thread about how integrating a woofer affected how an oboe solo sounded. When not properly aligned (I am talking microseconds off here) the oboe sounded quite bad. When the woofer was correctly aligned the oboe sounded phenomenal. There is no way I know of to detect this with our current measurement capability.
The Rockports certainly have articulate bass. Matching that is not trivial. It just depends on how badly one wants it.
The subs I personally want are from a company called PureLow. Look them up. Not cheap but integrating a pair of these would be pure bliss with any speaker. Unfortunately, getting a pair with the required matching amplifier, cables and crossover is going to cost almost as much as a pair of mains.