This is my concern too. The foam donuts would create an airtight seal, and I cant see that here. This dustcap seems to have been in place for years, its definitely not a factory job?
The foam donuts certainly do form a seal,and that was the manufacturers intention. The ''tube'' is bonded ( not just structurally,but in an air tight manner) to the pole-piece,which has a central hole to accomodate the bolt that secures the force cancelling rod. When it is all properly assembled,the driver is in a totally sealed enclosure.
Not having the proper sealing does affect the bass.Trust me on this (note the instrument I'm wearing in my avatar
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Not to be a killjoy,but I have seen the rubber surrounds come loose from the baskets,not because of glue or rubber failure,but due to the paint delaminating from the baskets.Go figure....
Also,pay attention to the rubber grommets in the basket mounting holes. They tend to rot and fall apart.Luckily,they are readily available from any good hardware supplier.
As for the slight lack of bass,you are correct. These guys start rolling off at 55 cycles,so the absolute bottom octave may feel a little ''light''. Mind you,this can be a very good thing in most rooms. When I have eq'd my guys back up to flat (using KEF's own specs) the bloody crockery keeps falling out of the cupboards.Just for farts and giggles,set a moderately/slightly loud level,then go to another room,preferably one downstairs if possible,and listen for a few minutes.See how fast you're running back up to turn it down