Rather than rebuilding the URL, just copy the code below and use Paste Link Information From Clipboard in the Managed Links Window for whichever link you are editing (or Add...to add a new one).
Amazon CDs & Vinyl: This link only shows up for Audio, and it only searches CDs & Vinyl
<Link version="1.2">
<Item Name="Name">Amazon CDs & Vinyl</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">http://www.amazon.com</Item>
<Item Name="Search URL">http:////www.amazon.com//s//ref=nb_sb_noss_1?url=search-alias%3Dpopular&field-keywords=Hexify([Search])</Item>
<Item Name="Filter">[Media Type]=[Audio]</Item>
</Link>
Amazon: This link shows up for Audio and Video, and it searches all of Amazon. I keep it as a separate link for Audio to find concert DVDs, merch, etc
<Link version="1.2">
<Item Name="Name">Amazon</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">http://www.amazon.com</Item>
<Item Name="Search URL">http:////www.amazon.com//s//ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=ifelse(isequal([Media Sub Type], TV Show), Hexify([Series]), isequal([Media Sub Type], Educational, 1), Hexify([Series]), 1, Hexify([Search]))</Item>
<Item Name="Filter">[Media Type]=[Image],[TV],[Video],[Audio] -[Media Sub Type]=[Home Video]</Item>
</Link>
Amazon MP3: I still keep this one too just to find MP3s (although I think I've only bought one once)
<Link version="1.2">
<Item Name="Name">Amazon MP3</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">[AmazonBaseURL]</Item>
<Item Name="Search URL">[AmazonArtistSearchURL]</Item>
<Item Name="Filter">[Media Type]=[Audio]</Item>
</Link>
AMG: Not sure if this is the Default AMG link (I've modified my links quite a bit), but this is what I use.
<Link version="1.2">
<Item Name="Name">AMG</Item>
<Item Name="Action">1</Item>
<Item Name="Base URL">http://www.allmusic.com</Item>
<Item Name="Search URL">http:////www.allmusic.com//search//artist//Hexify([Search])</Item>
<Item Name="Filter">[Media Type]=[Audio]</Item>
</Link>
You'll notice all my links Open in external browser because I think IE and Chromium run too slow within MC. You can easily change that after pasting the code by using the Action drop down menu in the Manage Links Window.