It's possible that your alnico magnets need to be re-charged. Alnico, especially if dropped or played at extremely loud SPL's, can lose some of it's magnetism after 30 years (I hope I'm never told I've lost some of my magnetism in the last 30 years ;P. You also mention in your post that "I don't know the history of my Altecs i bought them used in Norway, but bass membrane and surround looks almost new (or too new for 30 year old speakers)." Maybe they were re-coned with non-Altec or non-GPA cones. Posting photos of them here may help to determine if they are real and/or original Altec cones including photos of the backs of them which show the number printed on the backs of the cones......