Thanks Tom. I suspect that is exactly what it is. Is that less likely to happen with a sealed sub?
As far as what I am looking for, it might involve more headroom in mid and upper bass. I don't cross it over at an extremely low frequency. Usually in the 70 to 80 Hz range. Even though the Thiels are supposed to be flat to 30 Hz, they eat power like there's no tomorrow and keeping the crossover reasonable seems to help. Plus, I do like the sound slightly on the "meaty" side. What I'm looking for, in general terms, is something with at least as good extension, but cleaner for music.
I wouldn't say the subwoofer "order" (sealed, PR, ported, etc) would itself be more/less prone to "audible noises" when driven to its limits. The key is design refinement and to a large degree component quality. Give us your plus/2 design and allow us to "refine" the electrical side of things and bit and I'm sure we could minimize the noise you are hearing and improve the sound quality. The designer responsible for your plus/2 performance did his best....but he has learned quite a bit in the last 10 years..
All of the Power-X subwoofers use advanced DSP integrated amplification. This allows much great precision with the system limiters, compressor, attack, release, etc, etc. There are no performance metrics for this yet so I would say the best source of information would be subjective comments from users and reviewers.
Which "tuning" mode are you running with the plus/2? (how many ports open, switch setting on the sub?)
Power Sound Audio