Can you damage your speakers by plugging bass port vent?

I have ProAc D40R and plug the bass ports. I used a couple sheets of bubblewrap, folded over and taped. It works great to control a little bass boom, which is a function of the speakers position in the room and my seating position relative to the speakers, not some anomaly in the speaker's design. 

@ether

no damage for sure but be aware that not all ported speakers are designed to be plugged. Unless your speakers came with plugs originally chances are they are not. It depends on physical properties of the woofer suspension stiffness and travel. In general sealed speakers are looser and have more cone travel and ported speakers tend to be stiffer with less travel. Some are right in the middle and capable of both modes.

Plugging speakers that were not designed for it may restrict the travel of the woofer such that the output will suffer. 

Such was the case with some PSB towers I had.  Too much bass so I plugged the ports but the sound was not good.  See if you can give yours more distance to boundaries if plugging doesn’t sound very good. 

Try a Schiit Loki Mini and turn the bass down.

The only way I could get my R11’s to play nice with my F12SE’s was to plug my R11’s two rear facing ports with the KEF supplied foam inserts. The plugs lessen the bass, but don’t seal.
Frequency response changes verified with room correction software. Then used mini DSP on sub to further flatten. Very good results. 

+1 dynamiclinearity. The "ported speakers have slow boomy bass" is a proverbial old wives tale.

@OP, with the K6 being ported through the bottom of the cabinet, I'm not sure that you will find an improvement overall - yes you can reduce the bass extension, but you will change the overall tuning of the loudspeaker and probably not in a good way. Personally, I've never found that plugging the ports in loudspeakers results in  net overall improvement.

You would be better spending some money on some bass trapping and some absorption for the first reflections. Acoustic treatment is cheap and you can still use it when you move somewhere more permanent.

All that bass energy, builds and builds and builds until suddenly, "KABOOM!!"

Re panzrwagn - Closed box sound is dryer because it ends faster, has less overhang. It's more detailed because it stops more quickly. Speed of bass is really not the initial attack but how quickly it stops and closed boxes stop faster. It's just that the extra overhang of reflex boxes adds richness like amps with high second harmonics that people like despite being less accurate.

With modern programs there are good reflex loadings(albeit less good than good closed loading). If reflex bass is too boomy it's either incompetent design or often unfortunately achoice to make a speaker sound bassy because that sells in way too many cases.

A ported speaker will roll off at 24 dB/Octave below it’s tuning frequency. A sealed box rolls off at 12 DB/Octave, typically a half octave higher. An optimally tuned system that’s flat to 40Hz will be -24 dB at 20 Hz. The sealed box will be flat to 60 Hz, -12db at 30 Hz. Ported systems are harder ( but not impossible) to tune properly and so their reputation.for boomy ill-defined bass. Sealed boxes reputation is for tighter but ’slower’, less lively bass. It’s way more involved than just sealing the box. But if you understand the basics it makes what you hear so much more comprehensible.

No

You can manage bass with partial plugs such as socks or a towell

fully plugging is a bit extreme.

Every one is correct. There will be no harm done. But you will, of course, lose some of the lowest bass since that comes from the port. How much loss depends on how much you stuff. Experiment; try using various amounts of say Dacron or plastic foam, etc.

If you stuff it so there is no perceptible bass from the port you will also improve the Q of the bass; what bass you have will be tighter.

An old, fine example of a stuffed port is the Dynaco A25 loudspeaker, the best selling speaker of all time. It used a stuffed port and while the bass was not subterranean, the quality was excellent. SEAS of Norway built it for Dynaco and have designed a modern kit version, the A26, which Madisound sells as a kit. 

Plugging bass ports is a totally normal move if you're in a too small or bass compromised room go for it.

Thanks all....was concerned because there seem to be a lot of energy coming from that woofer....at normal listening levels you can feel the bass in that room.

I have seen those plugs with monitors but wasn't sure this is OK for larger units.

My old B&W CM6S2 came from factory with a small selection of foam inserts for the ports to adjust bass. 

But if it sounds horrible in your concrete room, I doubt that plugging the ports will solve the issue. I would rather consider near field listening to take the bad room out of the equation. 

The sound might not be optimal but won’t damage your speakers

Wasn't there a "Macgyver" episode where he did that and improvised a bomb explosion?  I'd have my Swiss Army knife ready.

I have had more than one pair of speakers arrive from the factory with foam port plugs.  It will not hurt the speakers , it will be fine.   May not sound that great but will not hurt them.  

No, you will not hurt the speaker unless you jam something so far inside that you physically damage a driver or the wiring. Some ported speakers were sold with foam plugs for controlling bass response.