Speaker problem

If your amp continues to function, with the center channel speaker disconnected, there's an obvious problem with that component.  There are a number of things(within a speaker system), that could result in a short.  ie: a burnt voice coil, if the coil wire doesn't burn through(open up), will have lost it's insulation between windings, drastically reducing the driver's impedance.   That could be triggering your protection circuit.   If you have a multimeter, check that unit's impedance, or- try another speaker, in that position, eliminating the suspect component.

AF-what speakers are you using?  Yamahas do NOT like running less than 6 ohms into multi channel.

Today I am using a NAD m17 as preamp + classe sigma amp5 as amplifier and the speaker is a HTM2 from BW - 8ohms

Did the Yamaha/center speaker combination work, at one time? If so- refer to my first post. If not, is the NAD/center speaker combo working?    Clarity, please.

Sorry, the speaker was disabling the amp channel no matter where. It did disabled the amp channel as front channel as center channel. It did disabled the amp channel at channel 1-2-3-4-5-6-7. The problem followed the speaker. Tried also changing cables from pre amp to amp and speaker cables

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.

Do any of your speaker wires run under a carpet? I’ve seen many cables shorted(their insulation crushed), after being stepped on repeatedly.

I do have a multimeter. I can test that. Wiring only the midrange and the Tweeter and then the bass. Do you want me to test the resistance while playing? 
No, it is not over the carpet. It is on the furniture. No, it does not make any sound before it goes down.

Remove all wires from the HTM2(jumpers and from the amp), and check from red to black, one binding post pair at a time, multimeter set on, "Ohms". If one pair shows a short, or- very low impedance, you’ve found your issue and can stop. See what you get there, before trying the woofer and tweeter, connected individually to the amp(one at a time). REMEMBER: volume all the way down, and barely/slowly bring it up, especially when trying the tweeter.