Im getting my older SI reconned since its blown. My question is, whats the BEST for a 15" SI sealed? What airspace before displacment would yield best results?

2.5-2.9.

2.5-2.9.

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Hmm my current box is at 2.5 after displacement.....I wonder why the coils blew. After I went from ported to sealed, is there anything I should have changed on the amp? Turn the SSF off?

Just because you are within enclosure specs doesn't mean you won't stress the driver. Depends on power output, clipping, etc..

You don't always need RMS to push the driver beyond it's limits.