Why are my speakers resonating????

Hi and thanks for your time. This is my first post although I have been with Audiogon for many years. I have a pair of MB Quart S802 speakers a few months ago and with certain lower frequencies of music I.e., piano, acoustic guitar and female vocals I have one speaker that resonates at times during these lower notes? These are replacing a pair of 1995 MB Quart 502s I purchased new and they never gave me anything but pleasure. Nothing back then came close for their price and I sampled a lot of speakers. I never understood how anyone could say their "too bright" either. One person wrote it in his review and it still sticks today. I run all Acurus amps and their AC3 processor, an NAD 565BEE c.d. Player along with all Audioquest connectors, NRG5 power cords, CV8 w/72v spkr cbl,, King Cobra audio cbls and VDM5 digital cables.
Any help is very much appreciated. I do like the Quarts. I contacted them, now Maxxaonics?, and their not able to tell me anything because they purchased Quart after the 802s were made in 2006. And yes I wish I could get some Von Shweirts' or 802 D's even PSB but the last 15% of a restoration on a 1966 Ferrari 275 GTB4 my father and I were building and now I am the soul financier of it as he passed a few months ago.