Don't recall ever using R13 for anything, historically it was mostly R19 and a little R25 and R36 for in-wall and attic respectively IIRC. FWIW, my old cabs have R19 minus the paper backing since I was between the loss of Celotex and hadn't found a friendly 'tin knocker' (htg/AC duct fabricator) that would give me small scraps and sell me new sheet stock.
Anyway, I imagine R13 isn't too dense, especially if you can still hear any cab 'boom' using the click test or other tester that will do an impulse response: http://farm6.static.flickr.com/5019/...022c3b_b_d.jpg
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