I have a pair of M-Audio Studiophile AV-40 monitors and one tweeter is open circuit. The company got taken over and it appearsthe new owners have no plans to support these. Evevn though you can still buy these speakers in the shops there are NO replacement tweeters available! !!! There must be an equivalent out there , they are 1 inch tweeters , I really like the sound but I'm not buying a new pair with this support policy!