How do SVS subwoofers compare to REL?

When you consider the actual improvement it can deliver to an audiophile two-channel system (especially one employing small to mid-sized tower speakers or large bookshelf speakers), how little effort it will take on the listener’s part to gain that improvement, and that it won’t detract from the sound quality of the main system, it’s probably one of the best buys a subwoofer-less audiophile could make...

my experience leads me to suspect that few, if any, unbiased listeners would express a clear preference for one of these subs over the other.

There are differences and advantages to sealed and ported designs. According to SVS, "sealed subwoofers typically have a smaller overall cabinet size and footprint, allowing easier integration into the listening environment with minimal visual impact to the décor. A properly designed sealed subwoofer will typically exhibit less phase rotation, lower group delay, and reduced ringing in the time domain. These characteristics make the sealed subwoofer a natural choice for critical music applications, and are typically described by enthusiasts as sounding tighter and more articulate, with less perceived overhang.

A sealed subwoofer naturally has a shallower roll-off slope than a ported subwoofer. SVS takes this concept one step further by employing sophisticated DSP equalization to tailor the overall shape of the frequency response and roll-off slope, in order to take maximum advantage of available ‘room gain’ so common in small to mid-size rooms. The end result is much deeper in-room extension than the quasi-anechoic frequency response would otherwise suggest.

With each successively deeper octave, cone excursion quadruples in a sealed subwoofer in order to maintain the same sound pressure level. In addition, the equalization required to tailor and optimize the quasi-anechoic frequency response consumes amplifier power. As a result, a sealed subwoofer will typically have considerably lower dynamic output limits <40 Hz than a larger ported subwoofer in the same family/price range.  

In a ported subwoofer design, a relatively large enclosure size is required in order to achieve both a deep system tuning frequency, and sufficient port area to minimize chuffing artifacts at high drive levels. A larger enclosure also greatly enhances system efficiency in the deeper octaves, with no need for additional EQ boost to achieve naturally deep extension.
   
The result is 2-4X more peak dynamic output in the 18-36 Hz octave as compared to a sealed subwoofer in the same family/price range. This makes the larger ported SVS subs a natural choice for system applications with larger rooms (where less room gain is present) and IMAX-like playback levels, particularly on demanding Blu-ray action and sci-fi movies with strong LFE tracks."