Hi,
The CSS would be doing source NATing if you have a group configured with "add destination service".
This is needed to prevent asymmetric flows, but if the servers are directly connected to the CSS, there would be no asymmetric flows, hence you can get rid of the group and the CSS will start sending traffic to the server with original client's IP.
Configure the CSS as the default gateway of your servers to make sure they wont bypass the CSS on the way back.
Hope it helps!!
Diego M.