Lens suggestion - prime, or?

I don't know anything about your 18-70 and tend to doubt that a filter will so broadly detract from  all of your images.  It might, of course, be covered with a haze from lack of cleaning. Removing the filter or cleaning it (I like lenspens) is worth a try.  In terms of a replacement, I like my Sigma 17-70 os and find it marries very nicely with my 70-300.  It is 2.8-4 so not as fast as a fixed 2.8 but you get the added range.  I use it a lot at 70mm and it has very nice bokeh.

--
www.flickr.com/photos/brev00