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. 
Thanks erik,

It's been a very long time since I have been the details of speaker design.  I burned out on the technical side of this hobby years ago and don't follow it near as much (that's one reason it's taking me forever to get around to setting up my subwoofers.  To do it right is such a hassle I never get around to it.  I just sit down and listen to my system as it is...)

I'd always noticed, though, that plugging the ports of a speaker seemed not only to tighten up the lower response, but also seemed to reduce the sense of dynamics, making it a bit more lifeless.  So that seemed to at least subjectively ratify that myth.  Perhaps that was just due to the loss of lower bass...(though I have both the larger and the slightly smaller version of the Thiel 3.7 and 2.7 speakers, the only difference being a 2" smaller woofer on the smaller speaker and not as deep bass, but it certainly sounds as dynamic or more than the bigger speaker with the lower bass....)
If you want to experiment, go to a fabric shop and buy some Bonded Dacron(polyester batting).   You can then control the vent function, by varying the amount of Dacron(tight or loose wrap/longer shorter roll) in the tube.  1" to 1 1/2"  thick works great and most places sell it by the yard(cheap).
I found out something interesting. The muddiness seemed more than anything else traceable to the fact that the tweeters were a good 12-15 inches above the height of my ears while listening. I tried tilting the speakers down by inserting a shim on the rear bottoms. This helped some - but when I turned the speakers over on their sides - voila! Now that the tweeters are almost even with my ears, the high frequencies and mid-range really came alive.

I didn't realize tweeters of this sort were so directional, but apparently 15 inches off axis is enough to severely attenuate the highs. I still keep the ports plugged due to the closeness to the wall. Pretty well balanced now, although I may have done something to the imaging by turning the speakers over on their sides - more experienced members than myself may be able to chime in on this.
That will do it, and depends how far away.

You could also try turning the speaker upside down. Many are measured on tweeter axis, so this will restore the vertical relationship.

Of course, it only matters if you can hear it.