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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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.
This is a little off topic. But I’ve wondered if plugging the ports on speakers that do no come with plugs can damage them by causing excess woofer excursion than the speaker was designed?