ATU Rx attenuation on 80m?

Joe, KQ1Q

Flex 6300: if ATU is used on 80m with my g5rv (zs6bk variant), it tunes OK but apparently attenuates the receive level by one or two S units. This can also be seen as a lower S meter, lower audio level, and the waterfall display background getting darker. This is with AGC both on and off. Is this normal? Has anyone else see it?

This does not happen on 40m, 20m or 10m. However with this antenna the SWR there is typically below 1.5.

On 80m the measured SWR with my RigExpert AA-30 at 3,600 Khz is 3.4, which is expected for this antenna type. The ATU tune up says "success". 

I tried the same antenna back-to-back on 80m with my Kenwood TS-590S which can also have the ATU in-line with Rx, and there was no difference in received signal strength with the ATU in or out.

System details: Flex 6300 running SmartSDR 1.4.0.130
Flex operating mode: LAN remote
Windows 7 Home Edition SP1 64-bitMotherboard: EVGA P55 FTW (A72 BIOS)
CPU: quad-core Intel i7-875K @ 4.01Ghz, hyperthreading on, RAM: Corsair Vengeance 16GB DDR3 12800
System disk: 1TB Samsung 850 Evo SSD
Data disk: 4TB WD4000F9YZ 7200 RPM 
Video: EVGA nVidia GeForce GTX-660 2GB, driver: 348.88
Monitor: Dell U2412M, 1900x1200 resolution