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.