You can set SNAT to none on the virtual server. Leave address translation enabled as that refers to destination address translation. As long as the web servers' default gateway is set to the BIG-IP, that part will work. If you want to allow outgoing connections from the servers through the BIG-IP, you can configure a forwarding virtual server (IP forwarding with SNAT enabled). If you only want to allow traffic from the web servers to pass though this VIP, enable it only on the VLAN they're on.