It now sort of works, though it doesn't look very good. I think what was happening was I was trying slightly different things and
xrandr needs the user to remove their traces before doing that. So good practice (for anyone trying this) might be to reboot X and try again, if something doesn't work:
I suspect that the original hardware problem I had (moribund motherboard) was compounded all along by some defect in the monitor, which means that apart from the aspect ratio, the lower resolution I'm getting without fiddling may actually be the best compromise. Slitaz, my current guest OS, only provides 800 X 600
and that doesn't look at all bad either.
I'll test it at some point with a bigger and better, and above all, newer monitor, and post the result. The third possibility is that this hardware-identical machine has the very same motherboard tuberculosis, just less advanced.
A footnote:
cvt gave me very slightly different output:-
Code: Select all
vanilla[~]$ cvt 1600 900
# 1600x900 59.95 Hz (CVT 1.44M9) hsync: 55.99 kHz; pclk: 118.25 MHz
Modeline "1600x900_60.00" 118.25 1600 1696 1856 2112 900 903 908 934 -hsync +vsync
which also worked; I couldn't see any difference.