If different step attenuator settings don't change the level displayed, chances are high that either the AC/DC input selector relay is broken (in front of attenuator) or that something further in is toasted.
You might do a sweep from a few Mhz to 1.5GHz and see whether that changes anything (level should get progressively better if input selector relay is dead, since impedance of broken relay (capacitive coupling) decreases with higher freq. 8568 has a coax RF limiter diode in front of input. It probably gets fried first, if input is overloaded. Remove it and see whether it changes anything, but install new one if it's broken before use, so the front end mixer lives longer. I have bought and rebuild a few 8568s and none had a broken input mixer.
BTW, if you dial in 0dB into attenuator setting, you can basically remove any attenuator elements from the equation. Also that noise free baseline IMO doesn't look promising. Didn't see you are in 1db/div, so that is normal