I have found that 300 dpi on my scanner looks bad, so when I need 300, I scan at 600 or 1200 and resize it. 150 or 180 also looks fine. 500 is just as bad as 300. Somewhere between 500 and 600, is where it changes to needing more than one pass. What is going on, here, that lower and higher resolutions are better in keeping detail, than the mid resolutions?
I use an HP 5300C scanner. It gets heavy usage and one time I heard a clunk from it, just once. Otherwise, it has operated nearly perfectly. I have wondered if it could be because the glass is foggy at the top. Has been since new. Have read that it can be taken apart and cleaned. Have wondered whether the hase could be at the focal point of the 300 and 500 res scans. But then, I don't often use the top part of the scanner bed.