Thanks to the useful tip of using a program which sets up another screen where you can switch back and forth, I've managed to get some decent settings for PAL, HighGfx, HD720 and XTREME, all in both non-interlaced and interlaced modes.
Am using 800x600 output for PAL non-interlaced, 1024x768 for HighGfx, a custom 1280x720 for HD720 and 1280x1024 for XTREME and PAL interlaced (I like the look of small scanlines on PAL interlaced as well).
I have a weird issue though. If the Amiga has been off for a while, my monitor will not turn on with PAL output, i.e. during WB load or pre-boot environment. It only turns on when it's ready in WB with HighGfx. If I then start PPaint in PAL mode, I get the correct 800x600 output that I set for PAL mode and from here on it works fine if I reboot the Amiga and even if I turn it off for a little while. If I leave the Amiga off for a long time, it again won't show PAL modes until I go back into PPaint. Anyone else noticed something similar?