Let me reply to myself...
I did some testing. While connecting the RF FRONTEND module RX1 output that normally goes to the RXTX MODULE to another SA I found something strange. If I injected a signal to RF1, RF2 or RF4, its level changed depending on the input used, as expected (i.e. RF1 has a 17dB input attenuator). But injecting a signal to the RXTX module RFRX1 input, and changing the input (which should compensate the level reading for the attenuation on each input), the level shown in the CMU screen did not change while changing inputs. What was going on? What I did was run the Version Manager application for boards being replaced, and guess what, it all started working as expected. Someone replaced the RF FRONTEND module and did not run the application so everything was a complete mess inside the instrument (there happens to be a good reason for that replacement, see below...)
And that unexpected signal dissapeared (well, not actually but it moved down about 2MHz and the level changed to around -100dBm). And what is more interesting, its level now changed depending on the input used, by exactly the amount expected. For example, whe RF4IN is selected, it is 17dB lower than when I select RF1. And that is because the instrument is compensating for 17dB loss in the RF1 input to a signal that is generated AFTER the attenuator, so its level appears higher in the spectrum screen.
I also found very low output on RF3, let me open another thread for that completely different problem...
Thank you for your Help!
Regards,
Roberto EB4EQA