I agree with Bob, especially if it's one set of frequencies that causes an issue (most likely if the sub is well designed). But it also wouldn't harm a sub to have it's port bunged, so as a 'what difference does it make and do I like it' test then it might be worth a quick go.
On the second question, answer is probably 'it depends'. The bass will roll off at a higher frequency (all else being equal). However, this could allow you to turn up the bass level overall, so actually giving a lower frequency response than you had before - AFAIK that's how the multi-ported subs like some of the SVS cylinders work, though they may also have different port lengths. Whether that helps with a boomy bass depends on why it's boomy. If it's a room resonance then it might if you are moving the cut-off above the boom frequency ... but, my guess is that it's won't unless you have a very large room. But, as noted earlier, it would only take moment to find out.
SVS subs do lower their extension by blocking one or two ports. Their tubes are all the same length.
Single port subs will lose their extension if you plug the port. You will probably still have boom and no extension. So I agree with the others who suggest relocation or EQ.
When you seal a vented sub, you will NOT increase the excursion of the driver at the tuning point. It will, in fact, reduce excursion!
As an example (although not a particularly relevant one), my centre speaker is rear ported. It generates audible bass below its natural roll off point.. 33hz in fact (thats with an 80hz rolloff from the procesor!). I plugged the port with a sock and that stopped the unwanted low end and cleared up the dialogue a tad.
In short, there is no harm trying it out, but I doubt it will cure your problem.
Put another way, bunging the port is safe UNLESS you then try and get the low-end bass back by boosting the overall level. Despite the resistance of the air-spring caused by the nearly sealed enclosure, you risk driver damage.
I note that my earlier advice wasn't really clear on this point. What I meant to say was that if the booming has caused the setting of a low level of bass then bunging could allow you to tweak the volume level control up, and you might get lower bass frequencies, but at a materially lower output volume than you were previously able to get. It shouldn't be used as a reason for a massive rise in level to try and get the low-end bass back at the old levels of volume .. that way lies driver damage.
Thank's to all. I am in the process of trying to get my head around REW but to be honest finding it more trouble than its worth. As you may be aware the 309i (which I have although I realise I havn't made this clear ) has a noc filter. I have run the supplied set-up cd and the resulting graph showed a massive spike (105Db) at 42-48Hz I set the noc at the relevent mark and turned down the cut control fully (this goes down 16Db but the graph showed I actually needed to turn it down 20Db). The main problem is that I can tell immediately when the sub cuts in. There is no intergration between speakers and sub. Also the sound of the bass from the sub is totally different to that from my speakers. There are two ports on the 309i, seeing as I can't do any damage I'll have a play around with the bungs to see if there is any difference or improvement.
Any idea how selective the filter on the 309i is? It kind of sounds like you are reducing the spike, but with a filter that has a fairly broad range (technically termed a low Q value). Since you have dialled in a huge drop, if it is on a low Q filter it could play havoc with the rest of the sound. This is one reason the BFD gets good press - it has very very selective filters, so allowing a very clean notch filter to be put in (or wider if reqd). You should be able to see the breadth of the impact on REW traces - a low Q filter will cause a wide frequency drop in response. A high Q filter will cause more of a notch.
REW really is worth the effort if you can master its basics enough to get a subwoofer trace.
Ignore the MIDI side of things and concentrate on setting BFD filters manually and you reduce much complication.
Being able to see the actual response from your system within a few seconds is a hugely powerful tool in overcoming audible problems. It beats the old manual test tones hands down. Without REW you can hear the problems but don't have a clue how to solve them.
There lies the temptation of the SMS-1. This can mend a poor system response. But from the descriptions I've tread it needs manual attention to get the best results. I find the latter makes a serious dent in its value given its very high price. One should expect perfection at the press of a button IMO at this price level. No doubt its software will improve over time.
REW will tell you far more about your system at the cost of a £28 SPL meter. It does however mean you have to attempt the steep learning curve before the BFD can be brought to bear to mow down your peaks.