You will really like this reply!
Here's my story...
Generally, I purchase audio gear and it stays with me for quite some time. After 10 to 15 years, I get a desire to "see what has progressed over time" and I end up replacing equipment. As far as speakers are concerned...I am in my late 40's, so no, nothing really classic in my past.
I started out with my brother's hand-me-down Altec 8" 2 way speakers from the early 70's, and as a freshman in high school I purchased a pair of $500 EPI T/E 320 floorstanders that were very good. A few months after my purchase, they were reviewed quite favorably in a major magazine, so they MUST have been good, right?
From there I went to Klipsch Forte IIs and a pair of small Pinnacle PN-5s for a second set. they lasted until I went to...wait for it...everyones' favorites...Axiom M80s!!!!!!!
The M80s are my current speakers, along with the rest of my 7.2 surround setup. I am very happy with them, even though others here will find a million ways to cut them down.
Getting to subs...with the Pinnacles, I had a 12" Audiosource powered sub. It was really decent back then and mated well with the PN5s. When I purchased the Axioms, I paired them with a single SVS 16-46 PCi. This sub plays quite deep and is still a decent sub. For its time (circa 2003) it was awesome for the price.
I like all of my speakers, but decided that I wanted new subs to go into a new theater room I am building. The SVS will stay in the family room off the kitchen and for the basement theater I ordered a pair of Rythmik F25s for use with the Axioms.
getting to the OP's original question...
I do not have a working theater yet. As such, I decided to move the SVS into temporary storage and I installed ONE of the two F25s in the same spot that the SVS had occupied for the last several years. Because this system is simply a family room for watching TV and to provide casual music during other times, the Behringers are not being used to EQ anything, so I can definitely compare one sub to the other.
The comparison may not seem fair because the SVS is only a 12" driver and a 325 watt amp while the single F25 holds two 15" drivers and 800 watts of amplification. This really does not matter as much as you would think. The way I see it, it IS fair to compare these two and I will tell you why...
Neither sub is pushed to anywhere near its limits and the SVS really does dig quite deep, so bottom end is not as different as you might think. If I sit and listen to music (as I did today) at roughly 70dB, neither sub is pushed hard, neither sub is playing really deep (while the stuff I listen to has PLENTY of musical bass, I highly doubt that it goes much deeper than 30 Hz). The difference between the two can only be described as night and day.
I loved the SVS, it was killer compared to a lot of more expensive subs of the time. It plays quite deep and is fairly musical. Then the F25 showed up. Bass notes became clearer. The pluck of a bass guitar was not simply more realistic, but it now has an air to it. This new found "presence" can only be described as more revealing. It is like the difference between hearing a kick drum on a mid-fi system with decent speakers and hearing the same exact song from the front row of a live concert. The F25 is just plain more articulate.
OK, so this is an F25 vs a decade old SVS that cost about half of what the Rythmik did and has a single 12" driver vs dual 15 inchers. This is not about max SPL, this is about the sound at my normal listening levels in my everyday family room. Neither sub was pushed. I could also mention that I have heard other subs that cost MUCH more than the F25. They were in different environments, but in the end, they did NOT sound as tight or as musical as my F25 does.
So...I was long winded and not to the point and I am goofy enough to list my other speakers as Axioms (and coming from Klipsch Forte IIs besides!!), but I will tell you that I wanted a Submersive and decided on a pair of F25s instead and I am glad I made this decision. While I can't say that the servo makes the F25 sound better than the SVS, I can tell you that at modest listening levels it is simply much cleaner. I can also tell you that other expensive subs that I have auditioned did not seem to be as clean or articulate either. Just like the GTGs, where people commented on how good the Rythmiks sound, my experiences with my F25 and what I have heard from other subwoofers leads me to the same conclusion. The Rythmik F25 sounds very clean. Cleaner and more articulate than other subs I have listened to.