Well, after a few weeks of direct testing and comparison, I have made a final decision.
I listened to many different movies as well as a lot of music and in the end I found the notes I was taking on the differences between the two subs to be very similar to what Mojo experienced. In my room, I found the 10hz tuning (with two plugs in) to give the flattest response, and by far the best extension below 20hz (which is to be expected given that this is the design of the port plugs)
At extremely high volumes, I found the SVS to be capable of playing slightly louder and deeper. The notes that the SVS hit were in many cases tighter and almost always had less lasting resonance. The lasting resonance from the 600 is what I believe creates the "warm" sound that a few others have described. It is indeed warm and enveloping. Envelopment is the best description I can give for the general experience I've had with the 600.
In some cases, the ability of the SVS to hit harder and without resonance was nice, but in most cases, I found that I preferred the warmer Axiom sound. The few exceptions to this (and in these cases they were dramatic) were on source material that hit extremely
- the server room scene in the movie "The Pulse." I believe this is ~10-15 hz sound.
- a select few of the deepest rumbles in U-571's depth charge scene
- the War of the Worlds pod emerging scene where they fire their lasers into the running crowd.
On these scenes, the SVS was clearly dominant. It hit low and monstrously hard. Both subs had a downside on these scenes, however. The 600 consistently put off a strange rattling (almost a chirping) sound in these scenes. This could only be heard in moments of extreme bass that were not surrounded by other noises. The Pulse scene is a perfect example... it is loud, low bass, with almost no other ambient noise. A very challenging scene.
The SVS's downside in these scenes was that because it does not have a DSP controlling it like the Axiom does, at high volumes it "bottomed out" and the distortion from the driver overextending was obvious even over the extreme volume of the rest of the sound from the movie scenes. Choosing either this or the "chirp" of the Axiom driver is a hard choice... The trick was driving the SVS just to the point of distortion, at which point it produced amazingly good sound.
When not listening to scenes with abnormally low and loud bass, I found the results of the two subs very hard to distinguish and honestly am not sure whether I would be able to tell a difference in a blind test. A possible exception was running the SVS in sealed mode (all plugs in) during which I noticed a bit more "slam" or "punch" that we've been talking about in the past. It was subtle in most cases, and for a few songs, a lot better to my ears.
I also had two other buddies here for the weekend during which we spent 2-3 hours up here doing A/B testing. To their ears, in blind testing, they preferred the Axiom to the SVS in about 8 out of 10 tests. Of the 10 tests, I preferred the Axiom in 4, the SVS in 1, and on the rest could not tell a difference.
I purposely told them nothing about either sub in advance, and interestingly, their comments too focused on the "warm" surrounding sound of the Axiom (or the "brown sub" as they called it) versus the "black sub." They occasionally described the SVS sound as "flabby" compared to the Axiom which I thought was interesting. I believe what they were hearing was the distortion that became evident at higher volumes, but am not entirely sure. Steve (WhatFurrer) mentioned this a few times as well while he was here the week before comparing the two.
So.... in the end, what did I decide? Well, I think it should go without saying that it was not easy, and that these both are truly amazing subs. If I'd never heard both subs side by side, I would gladly have kept either in my home theater and know that I'd have been extremely happy with either in the long run.
In the end though, I decided on the Axiom. The main reason? Surprisingly, in the end, because I didn't notice a huge difference in performance between the two, one of the main decision factors was aesthetics of the Axiom versus the SVS. (I can almost hear the whispers of "heresy" from the crowd when reading that after all this work, the looks of the sub were a deciding factor!)
The Axiom is an attractive box that matches the finish of my mains in high gloss cherry and also takes up a lot less floor space. The SVS is a plain (albeit well-constructed) black box that is just plain ugly by comparison and is a lot harder to hide in the room. SVS came and picked up the PB13 today for return shipping, so my sub comparison saga is at an end. It was a fun series of tests to perform, but in the end, I have to say that I'm glad it's over.