To adjust the bass and treble on the Sony STR-DH590 AV receiver, you can use the equalisation settings available in the device menu. Here are the general steps:

1. Access the menu: Press the "MENU" button on the remote control to open the main menu.
2. Select "Audio Settings": Navigate to the "Audio Settings" or "Equalisation" section and select it.
3. Adjust parameters: Once in the equalisation section, you can adjust the bass and treble levels to your preference. Usually, there are options to adjust bass and treble independently.
4. Save your settings: After making adjustments, be sure to save your changes to apply them.

These settings will allow you to customise the sound according to your preferences.