if the subs are new then they are being broken in pretty well, that smell is the voice coils burning, but there is a different type of burning that can be done. If you have had the subs for a while then that is a very bad smell and you need to turn down the gains. If the subs are new then you need to take it easy for a while till they are good and broken in. If they are new it is common for them to smell, but its not really good for them