I don't know what the answers are in this debate, I mean, discussion, but I suspect that the '...how low, how loud at what % THD...' is a very small slice of the picture.
If I built relatively large ported subs that extend low, are loud and measure well as far as THD numbers in lab-like conditions, I'm sure I would say that these are the desired criteria.
Ported subs are not accurate. Anyone who has ears and has compared knows that. Being that all explosions, gunfire, power surges, etc., etc., in movie soundtracks are sythesized, there isn't a way to know if it sounds accurate or not, so how low, loud and 'clean' it is seems to matter to most average listeners...for now. But, for music reproduction, saying that a ported sub is 'accurate' (assuming that means a flat resonse graph) most certainly does not mean it sounds good. There are many types of distortion. THD is one type, but certainly doesn't prove a sub's distortion status.
I think the criteria should include features like phase adjustment, crossover, delay, amp specs, EQ specs, limiter specs, signal/clip indicators, system 'Q', etc. I would also like to see some sort of descriptive scale to describe the sub's sonic signature, which every speaker ever made has.
The THD numbers are taken with very contolled signal input/output at various frequencies. Real listening is nowhere near this arrangement. The transients in movie soundtracks and some audio only MC software are radically huge. They cause the preamp to send a clipped signal to the amp or a hot enough preamp signal to clip the amp. How would the sub's owner possibly know this with no indicators? That person's RS SPL meter, set to C weight, fast, is only showing 109 Db, so he/she thinks it's definitely 'clean' and well within the limits of the powered sub's capability because of the published test results. This is only one of many scenarios.
Corner placement of a sub is, generally, the best placement, but where in the corner? Too close can be bad, too far away can be bad and off center can be bad. Two subs is better, but not if they are different distances from the LP and not time corrected. These facts are rarely addressed when someone like TV or TN says 'corner placement'. Corner placement should have given parameters.
These are just rambling thoughts. As I said, I don't know the answers, but I don't review subs or tout test results/placement choices, etc. as criteria based on which anyone should purchase a subwoofer, so it isn't my place to know. I'm only pretty darn sure that a standard criteria list shouldn't be very hard for these industry leaders to come up with and adhere to. I'm disappointed that they haven't. Instead, speaker specs and tests and graphs mostly serve to confuse the newcomer and allow too many inferior products to swamp the market.
I wonder if TN has rethought his SACD/DVD-A comments which were mentioned above and along the lines of 'they cook the levels and tweak the EQ so that people will be fooled into thinking they're 'better' formats.' I believe this appeared about a year ago. I have no idea if he actually said or believes this. I hope not.