My 40 - 6m End Connected Windom does not need re-tuning on 40/30/20 when used with the KX2 and internal ATU:
The SWR varies from 2:1 to 4:1, as there is no impedance transformation with this type of Windom. Nevertheless, the KX2 handles it no problem; press ATU once (not on the activator’s QRG) and forget about it.