Hi,
I have refoamed many,many cx's , both cx50 and cx100's without too many problems.
The tolerances are fairly small on these (rather small) drivers and even though they don't seem to scrape at first they still might do so when hitting rumbling lows. I play someting like the song 'Angel' from Massive Attack's 'Mezzanine' Album to discover if there is some distortion or not.
The cx50 are not really the best speakers (they are just cut-down cx100's), though they are sufficient for small spaces and excellent in home-theater use. Not really that great for listening to music. (if compared to Penta's they will for sure dissapoint...)
A sub does (even a little cona like I have) help, but switching to cx100's really is a big improvement (necessity...imho) for listening to music.
I found changing the cap in crossover does ihelp a bit as well. The last pair of cx50's I still have now use a new 10$ 'generic' kind of crossover from Boston acoustics that really helped to push them a bit further, especially at higher volumes with nicer lows and a fuller midrange.
Yes, they need some time to play and 'loosen up' the refoamed drivers as the Good hifi foams are fairly rubbery. The last pair of cx100 that I refoamed (the chrome-plated ones) are only now coming into their own having lost their intial pitchyness of the bright highs and they have been in use every day. I did try some softer foams from an North-American supplier that turned out to be total crap. They were too small and after finally getting ones that fit, they were so thin they ripped after a month. There is pressure in those cabinets...
Do not forget that the 'new' ones you buy at the B&O store are not necessarily as 'new' as they would like you to think..... spare parts do age as well.
Hope it helps ....