The Wharfedale cloth surrounds were treated with a black rubber compound, and in many cases a 100% seal was not achieved. There are flexible sealant coatings available from many sources that will fix this small problem, although I have a preference for dimensional fabric and craft paint, Black Shiny No. PM 101,originally obtained under the 'Polymark' trade name which is now known as Duncans. However there can be other reasons for the bass shortfall that you are experiencing and I suggest that you check for the following issues:- a creased cone, braided pigtail leads that have frayed , the paper voice former has crumpled or the voice coil that has partially unwound due to adhesive failure. These are vintage speakers and they do require remedial attention from time to time to keep them in good order.