Your suspicion that the midbass driver is blown may be well founded. I had tweeters replaced in mine recently, so I guess parts are still available.
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I've had the same issue, usually with loud piano passages. Others on the forum have stated similar problems usually with the 3.5 midbass. I have contacted Infinity several times and get the usual "we don't receive any complaints about that speaker". Guess I will try snail mail. You can try this link, it looks like they have parts.