I contacted the Swedish company that services Infinity (and JBL etc.) and amazingly they just had another MTS speaker in that had the same problem with thew 3.5" driver, sold at the same HIFI dealer as mine. The service man showed me the reason it sounded bad (same reason as stated earlier in this thread), and that it was clearly a manufacturing problem, probably due to magnetic material being present when they made the driver.
So he replaced both the 3.5" (left and right speaker) at no cost! "When you buy Infinity you should get good service, and these were faulty drivers from the beginning ..."
He had never come across this before and now he got two customers the same week!
Anyway, now that new drivers may be ordered you might as well have the speakers serviced if you have started to hear som buzz - it will get worse, believe me!
/Magnus