DVDs cannot provide as good an image as BDs can: they're limited to a maximum resolution of 720x480 for NTSC source material and 720x576 for PAL source material. Too many of them are not even that good. I've seen some that are worse than VHS tapes.
There are BD players available which provide a variety of video manipulation options which can be used to improve the appearance of DVDs, but unfortunately the S590 is not one of them
You'll need to upgrade to a Sony BDP S790, Panasonic DMP-BDT 500 or an Oppo.