Spin, I'm just in the middle of 'saving' a lot of glass plate negatives and positives.
Like you, I started out looking at them and thinking 'they look pretty good' only to find that no matter how I tweaked my Epson photo/film scanner the results were almost always dire.
What you need to do is rig up a lightbox or similar and photograph them again.
Here is a first draft of a positive:
and here is a first pass glass negative:
I've no idea what processes were used to create these slides - they are believed to come from the 1910s to 1920s and some of them are a bit touch and go, but I've managed to get a reasonable facsimile from all of them by re-photographing them.
Good luck with the restoration process Spin