So, I guess you do not measure flux if are getting. You might measure sensitivity and we might presume what when perm magnets has the same sensitivity as electromagnets then the drivers have the same magnetic flux density. Let also presume that when you report that “at 100 volts sounds almost equal” then you have equalized flux and the drivers output. I think you do not see a lot of HF restrain with electromagnet because the JBL 2420 is not so strong HF performer and it rolls off relatively fast, right after 9K-10K. So, you drive your electromagnet with more voltage (in fact with more current) that yields more flux and to “stiffer” sound. Then you wait until the driver warm up and soften sound to the level you like.
Here is in my view is the most interesting thing happen. I am in contrary to haralanov do not deny the proposal of the be’s friend that heat might do magnetic benefit (softening). The key question would be in my view if heat induces mechanical benefit of magnetic benefit. If we were are taking about the drivers like Vitavox with new diaphragms or Goto or any other driver what a metal cone surrounded with softer syntactic suspension then I would undeniably felt that the temperature-susceptive plastic suspension doe warmed and change sound. However, the diaphragm in the small JBL 2420 driver is full metal (correct me if I am wrong) and there is absolutely nothing in that driver that might be soften mechanically. So, there is a very high possibility that a hot magnet radiate some kind of different flux that makes some kind of difference. That is the qestuion that I would like to keep as opened question.
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