Darken results in degraded image

I have a test, on jsFiddle (uses the browser version of 0.5.6), https://jsfiddle.net/niccih/mdftLojv/24/, where I am trying to darken the image. As can be seen the result (second image) is not quite what I was hoping for in that the quality has degraded noticeably.

Any advice as to what I may be missing, or what alternative steps I may take will be greatly appreciated. Note that the 'brightness' example is just for my own reference and is not the result I am looking for. Other image manipulation packages seem fine when darkening the same image, but not being browser based, they are not a viable option (my requirements are dependant on being browser based), hence wanting to use Jimp.