Does Plugging Bass Port Affect Pace, Rhythm and Timing?


Hi, I recently purchased a used pair of Monitor Audio Silver S1 bookshelf speakers for, well, my bookshelf. :\  As expected, the presentation is a little muddy, and the highs not as extended as they might be otherwise on these speakers known for the quality of their tweeters.  I am about to try plugging the ports to see if any improvement could be had. I was wondering if there is any downside to plugging the ports on a bass reflex design as far as bass accuracy and speed. 
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Mine are about 6" from the wall right now.  I'll try moving them out 4 more inches first before plugging the ports to see what that does. The only down side is esthetics - having the front of the speakers out over the edge of the shelf. But I could learn to live with it if the sound improved dramatically! 
OK, I will do that, and see how it works out. I don't have the original port plugs, but I do have some large chunks of foam material I could use (or just go with a pair of socks). 
Are you using any type of footers? Lifting the speakers off the shelf will really tighten up the bass.