Here's some more information:
as suggested, I run
"No errors or warnings." and
device `hpaio:/usb/Officejet_Pro_8500_A909a?serial=MY922220FX' is a Hewlett-Packard Officejet_Pro_8500_A909a all-in-oneand
xsane-0.998 (c) 1998-2010 Oliver Rauch
compiled with GTK-2.24.10
with color management function
with GIMP support, compiled with GIMP-2.6.12
XSane output formats: jpeg, pdf(compr.), png, pnm, ps(compr.), tiff, txt
In Xsane docs it's said that:
This "slider" was available before but not anymore!
Select resolution that is used for scanning. If the backends makes available a range of resolutions (e.g. 100-600 dpi) XSane can display a slider or a list of resolutions. You select this via Preferences/Show resolution list
The backend can define a list of resolutions insted of a range, in this case it is not possible to enable the slider.
If I have understood correctly, I have external backend "hpaio" which comes with HPLIP. Support status for "HP Officejet Pro 8500 All-in-one Printer - a909a" should be "good" and according to this scanning should be "supported".
I do not know why I can't scan better that 300ppi. Scanner should be able to scan 4800 x 4800 ppi.