you don't have to use ABR plates on RL60 speakers, they were a design failing that B&O resolved 10 years later by offering an "RL 60.2 Bass Port Kit" part number 8480128 for about 100 euros, the kit basically converted the old RL60's to the later RL60.2 design, sadly the kit is no longer available from B&O
However, Mike at Speaker Hobby has modded his RL60's to great effect
http://www.speakerhobby.com/bangolufsen.htm
So that could be a better bet long term to consider !
Also, do remember if you do any work on the RL range of speakers to upgrade the caps to 250 volt polycarbonate capacitors in the filters, they sound better for it.