Mass-loading is something worth experimenting with, especially if you have problems with bass boom in your room (not meaning to sound like a rapper, it just rhymes!) - but things can be taken too far by overdamping.
Speakers are designed with the cabinet as part of the sound, and removing too much of that cabinet character can hamper the sound rather than help - I remember some very inert concrete speakers being made in the 80's/90's - they didn't sound too good! The Neo's were designed without the mass-loading so under normal circumstances will sound best this way, if you get too much overblown low end the option is there for you to tailor it, best so in stages as suggested by Andrew.
If you're happy with the sound then it's usually best not to change things, or if you do then leave the bungs out as mentioned so the mass loading can be removed easily afterwards (what you use to mass-load is another topic all in itself, the speakers we've supplied pre-loaded in the past used metal filings). If however, you happed to have sealed the bungs in place and can't get them out (fortunately not the case for the OP!) then don't worry too much about damaging the bungs to get them out - I've even had to drill some out in the past. Leaving the cavity open will have no discernable effect on the sound as the chamber is sealed internally. If the cosmetics of the open hole bother you, you can always give us a call to get a new set!