A few years ago a friend of mine built some ST Scart cables that were higher quality than the ones that are advertised on eBay etc.
I would first check that the scart cable itself is working without considering the scart to hdmi adaptors. It should work easily with the Samsung monitors. I have a Samsung SyncMaster 711MP and it works with scart cables wired with RGB, Composite and Commodore DTV with composite only etc.
What you could test is if the scart cables are "fully wired". By that I mean it should work if the video source supplies RGB and Composite wiring. A well made cable should also support the Status & Aspect Ratio up ( Pin 8 ) +9.5-12V on 4:3. If it doesn't support this you will manually need to switch to the video source by pressing AV1\AV2 etc. For the ST on the RGB lines you need 150 Ohm resistors otherwise the colour saturation is too high.
I take it the 800XL uses only composite output so it should work on any half-wired scart cable (composite only). I assume you have tested the 800XL by RF to ensure that normal video output is working?
Still got, still working: Atari 4Mb STe, 520STFM, 2.5Mb STF.
Hardware: Cumana CSA 354, Ultimate Ripper, Blitz Turbo, Synchro Express II (US and UK Versions).