Speaker problem

The center speaker has to be the problem, from your testing. It’s obviously damaged, one way or the other. Again, though- Did the Yamaha & HTM center speaker combination work, at one time(is this problem a recent development)?  You mention the amp shutting down, when you increase volume.  Does the HTM speaker make any sound before you increase volume?

The problem is recent noted. I am not using the Yamaha anymore. When I upgraded with an amplifier that it first appeared. When it first appeared I was using the Yamaha 2030 as pre amp and the NAD M25 as an amplifier. At near -20 DB the speaker was showing the problem.  They told me by phone that it could be the Yamaha ( I don’t know how) and I replaced the Yamaha for a NAD M17 preamp. It kept happening of course. I thought it could be related with some issue with amp/speaker. I replaced the amplifier with a Classé AMP5. You might be asking: why you did not sent the speaker to repair? - I did. But the mother f... of the repair shop charged me for testing the speaker and told that it had nothing wrong. That is why 

" They told me by phone that it could be the Yamaha ( I don’t know how)" How? Their cash drawer probably told them. There is the possibility that the HTM’s problem is intermittent. Again: You mentioned the amp shutting down, when you increase volume. Does the HTM speaker make any sound before you increase volume?

The HTM2's have bi-wiring provision.   Did you happen to change anything, regarding the wiring(jumpers or connections), before the problem arose?  

With the tweeter and woofer each having separate inputs, you should be able to check each’s network, by(with volume all the way down/jumpers removed) connecting your speaker cable to one pair of binding posts(red/black) at a time and barely/very slowly bringing the volume up. See if either driver has any sound. I’m guessing, you don’t own an ohmmeter.