I recognize this problem using sound test cd with the low frequency track.... in the beginning there was some vibration coming out from the cross-over back panel due to gasket worn out. i replaced in both cabinets and tested again with the same low frequency track with good result. i tried to increase little bit the volume looking the woofers moving good without any noise, but after few extra dB start to have some scratch noise coming from all speakers, woofer, mids and tweeters.. no any power guard alarm from ampli and the power showed by wu-meter was 20 watts... not both speakers left and right in the same time, first left and with one extra dB also the right.
Please note that the fuses has been already changed, and spectral balance control cleaned. With this scenario a start to think about the exhausted capacitor.
I measured the impedance of my LST and shoved on my Fluke instrument 10,4 Ohms: looking into cross over layout there is one 10 Ohms resistor connected in parallel of capacitor. So with this impedance, where is better to connect on my McIntosh output? 2, 4, or 8 Ohms?
Really many thanks for your comments, help.