Hi Ivan,
Yes, that is generally true (do not exceed +20 dBm input) for any combination of preamp/atten settings. It is recommended to stay under the reference level though if you can.
Also, a strong signal within the IF band but not within your visible span can produce IF overload.
The biggest concern is the input into the preamp.
This morning we made an additional note in our product manual that is related to this, for a different customer. See section 3.6
https://relatedcontentdemo.kinsta.cloud/sigdownloads/BB60C/BB60C-User-Manual.pdf
Let me know if you have additional questions or need further clarification.
Regards,
A.J.