First of all, I was referring to the SURROUND of the speaker, not the gasket that is clearly made of paper in case you decide to invert the speaker for something like an isobaric type enclosure. You're subwoofers do not have a rubber surround. They are from the early 90's and do not have a UV treatment and they are made of foam. Now that we are both on the same page about that, they are at the point where i am surprised your voice coil is still intact. Based on UV exposure, moisture, and time, non-treated foam speaker surrounds decay and erode. After this happens, and they are still in use, the foam will literally fall apart due to the vibrations. it is clear that this has happened to your speakers. You see, the surround (as you called it, "the rubber thing") is a suspension part. Just the like suspension on an automobile, it is there to keep the movement of the voice coil former traveling in a linear path while it passes the magnetic flux gap between the T-yoke and the top plate of the speaker’s motor structure. The reason that it must be linear is one of distortion. Any movement in the lateral plane results in physical distortion of the music produced by the vibrations in the air, not to mention when the former hits the T-yoke or the top plate, it makes some terrible scratching noises. Therefore, the surround of a speaker is directly linked to the fidelity of muisc produced from said device. So your argument about it not having anything to do with sound quality means that you know nothing about how a speaker works.
Second, the placement of a speaker in a listening environment is very crucial. Since it is of one’s best interested get the environment set up as best as possible to start out with the best possible results before relying on outboard digital signal processing (or analog depending on one's equipment). Even though the trunk of an automobile may not be flat, it still causes a time delay function on the front wave of the audio output of a speaker that causes a phase shift. It might only be a 15* shift, but looking at your setup I can come to a strong conclusion that you do not have any time correction software\hardware meaning you only have the ability to adjust the speakers 180* by flipping the polarity of the subwoofers at the amplifier or enclosure terminals. Phase is one of the easiest disruptions for humans to pick up on, specifically in the low frequency range, so to me it is the most important function to try and get right. It really is not difficult to hear or adjust for either. You say that this is unrealistic, but manufacturers sell time correction equipment that can be attenuated in the 1 millisecond range, this would be equivalent to moving a subwoofer about one foot.So you're argument about how phase adjustment being unrealistically hard shows that you know nothing about setting up a speaker system properly.
Lastly, just because you tried an alpine speaker, a Sony speaker, or whatever else (I’m sure in the same enclosure that you had your Rockford’s in) and they sounded crappy, does not mean that they are inferior to the Rockford subwoofers. Each loudspeaker has its own Thiele-Small parameters that dictate what kind of enclosure would be optimum. That means that the box you had your Rockford’s in is most likely not suitable for the other subwoofers. Try building a proper enclosure for something and you may be surprised at what new technology has to offer over old, overhung voice coil topologies from the 90's. So you're argument about your old speakers sounding better than new\other speakers is out because it shows you do not know anything about speaker enclosures or parameters.
Did I miss something?
Please do not disregard the knowledge of other people on the internet based on their post count. I was only trying to suggest something that could help.
Team DIYMA sound quality competitor