Marc,
In my opinion, if someone owns full range speakers and does not like the way the bass sounds, there are four possible reasons:
1. The speakers are not optimally set up in the room or the listening seat is not located in the general area for smoothest bass response.
2. The speaker/room match is not optimal. The room may simply not be able to handle the low frequencies that that particular speaker can produce. Acoustic treatment may help.
3. The speakers are not designed properly.
4. The owner did not buy speakers that match his set of preferences.
To suggest that all full range speakers be designed with adjustable low frequencies is overlooking the other possibilities which I mentioned above. You, Steve W., and Tang have very different conditions and to suggest that there is some trend toward better sound by turning off subs is a bit strange. I can see why large sub systems like Mike's towers have user adjustability built in to fine tune them to a customer's room. JL Audio, Magico S and Q subs and others have these too because the designer wants them to be flexible enough to properly blend in with the main speakers. But in these cases, the speaker/room (with treatment) match must also be appropriate.
A 3 or 4-way speaker is a completely different thing. In these designs, I presume the designer tried to integrate the drivers as well as he possibly could. It is then left to the customer to properly locate the listening seat and to position the speakers for the best overall sound, assuming the room is appropriate in the first place.
Have you experimented much with your speaker and or listening seat locations for smoothest/best bass response? It seems odd that after all of your recent efforts with system improvements, isolation, power, room, that you do not seem satisfied with the bass of your Zu speakers. When did you first start noticing these issues?
+1
Excellent advice; very well-stated.