Integrating a Sub into a system is not an easy task. I have found that a larger full range speaker capable of very low frequency response always sounds more integrated to me if properly placed in a room and maintains the proper phase alignment etc. as opposed to a small speaker integrated with a subwoofer. At least in a stereo setup. For home theater and the .1 channel its a different story IMO.
Obviously: your subjective experience is entirely valid to your subjective experience. But there are a few things I'd like to mention on your evaluation of objective cause.
For one: the phase of almost any speaker is not constant. The phase moves as the frequency moves. There should be no sudden shifts in a well-designed crossover; but 15khz and 150hz will not be in phase. It's OK, because absolute phase doesn't matter to the human brain and relative phase is only a problem at extremes. (plus both phase and delay to the sub can be adjusted in a decent AVR).
Indeed the Model T is already 3 different drivers integrated together. Adding a sub just makes it 4 drivers. (and that's not to pick on the Model T; this is true of any speakers)
Conversely: There are some real problems with using your mains for subs even if they are completely capable. To start with: good mains placement is also bad sub placement. They are close enough for comb filtering and along the same wall so likely to encounter similar null points and standing wave issues.
The choice to buy a sub or not for a 25Hz speaker is up to the listener. The SS8 also goes to 25Hz. The 802D goes to 32Hz.
Subs will go lower. Harmonics of some instruments will go lower. That's up to you.
