A long decay time at certain frequencies sounds very much like a room issue rather than electronics (otherwise a difficult speaker with inadequate power amp unable to control it, which I’m not suggesting is the case). If the new speakers go lower than the old that could explain why it wasn’t a problem before. You say you’ve looked at room layout - if speaker and listening positions are optimal and that does not cure then furniture placement and whether there is enough absorbent material. This thread may help: Speaker placement guide
Also: The Listening Room Reality
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