Elad FDM S2 radio has AGC that sets the gain automatically and adjusts signal level to stay within the dynamic range the of the receiver. If you turn off the AGC, The THRESHOLD adjustment sets the gain level. There is also a 12db attenuator option. See below:
SW2 MAIN FUNCTIONS FOR RECEIVING MODE | Elad USA inc.
I don't have any info on the Wellbrook antenna, but I believe it's fixed gain. It would be possible to use an attenuator inline.
My question would be, what is causing the situation where you need to reduce the gain?
I'm using an ELAD S2 and a Pixel Technologies RF PRO-1B Loop Antenna and only times I see an OVERLOAD indicator on ELAD is when there are close by lightning strikes and momentarily OVERLOAD indicator comes on.