I believe everyone is forgetting how important the room treatment is especially when it comes to low frequencies.
A rel T series can sound much better in a treated room than a rel S series in a regular room.
It doesn't matter if the subwoofer is very fast in a room that has no bass absorption --> You will hear the low frequencies reverberation instead of the actual sound getting from the speaker. Of course you will also hear the reverberation of the main speakers.
The dealers usually have treated rooms in which you can hear the differences between S and T series but at home the difference may be minimal and more related to price and placebo effect.
To make an idea: a subwoofer can sound 2 or 3 times better in a treated room than in a regular room, so save some budget for low frequency absorption too.