Thanks for all the good suggestions. Work has been disturbing my hobbies the last couple of days
I haven't had the time to dismount the speakers, take pictures of the membranes and remove some of the fiberglass so I only have top, side and back covered.
I have however been playing with the shelving pot in the JM filters and it does not have a huge effect, everything is a little more up front on the one extreme setting and a little slower/darker on the other extreme. I use the middle position for most of my listening.
If removing damping from the cabinets is not enough i might try closing the front port and cutting a port on the bottom (I have the 620's mounted on wheels so my wife can move them around) and see what that does.
Regardless of the missing bass output, the Altec's still sound quite good and are smooth, detailed and fast.
Regards
Kjartan