That is the same amp, 1800W vs. 2200W difference comes from the 4ohm/8ohm load. And maximum power doesn't affect the frequency response.For an "apple" to "more apples" comparison, it should have been
The single driver sub has the SVC 15" version of the TC-2000 in a 90L (net) enclosure (front firing). A single ~2200W amp was used to power it (4 ohms bridged).
The dual driver sub has two SVC 15" versions of the TC-2000 in a 140L (net) enclosure (opposing firing). Two ~2200W amps were used to power it (both 4 ohms bridged).
I don't know how much of a difference it would make, but if you can use one of the SVC drivers in the single driver sub and the 2200W amplifier, that may have some answers.
Just a thought.
We will try my SVC driver in my friend's 90L cabinet. But I'm pretty sure 20L difference doesn't do much.