There is no +/- figure so that 28hz is nonsense.
Goes up to 50khz?![]()
Just found out those elac are rated using this

Elac Vela 2.0 FS409.2 Floorstanding Speakers
The VELA builds on the highly successful and high-performing Series 400 while improving stereo definition and multi-channel audio in modern furnishings.

Frequency response (IEC 268-5) | Visaton
Which means +/-10db so I'd set those speakers to 50hz and don't correct under 50hz