CRT's are limited more by their horizontal scan rate rather than their vertical refresh rate, as they are analog devices.
A CRT with a max horizontal scan rate of 128 kHz can do 1600x1200@ 100hz refresh rate and may be capable of 2048x1536 @ 75 hz.
Your CRT has a horizontal scan rate of 70 kHz or 75 khz, which limits what it can operate at. You may be able to exceed the limits slightly but this usually comes at the cost of image quality, if you don't get an out of scan range error.
*REALLY* Old CRT's would actually physically explode (not like a bomb) as there were no cpu safety limits to exceeding the scan range. Those were the ones that would give you a scrambled screen if you went too far. Go even farther and...hello magic smoke !
Your 800x600 setting you listed isn't a default setting. It's the 'maximum' supported setting by the specifications.
800x600@109 hz puts you right at 69.981 kHz horizontal scan range, which is close enough to 70 kHz.
You may be able to exceed 70 kHz slightly but you get loss of image quality.
You should be able to make a CUSTOM resolution of 1280x960 @ 70 hz.
This puts you right at 70 kHz scan range which should not lead to any quality loss.
You can do it with ToastyX CRU, and use "Automatic: CRT Standard" timings under Detailed Resolutions.
If you don't get a blank screen or an out of range error, 1440x1080@60hz SHOULD also work.Edited by Falkentyne - 10/16/15 at 4:52pm