So they used the GPA kit listed on their website? Speaker Exchange | Altec 416-8A DIY Recone Kit

Full DATS specs please?

FWIW, I calculated the M-19?s net Vb as ~8.353487847 ft^3 using 416A/802.

Using the Margolis-Small T/S design routine, these specs equate to a ~0.8 Qtc using a minimal lining of 1? acoustic fiberglass insulation [AKA duct board], i.e. bottom, one side and back of the bass cab, though for the M19, mounting it at the top to isolate the horn cab as well as all sides inside it is what I?d try.

This is a slightly under-damped alignment though, which more often than not is acceptable to most, but I recommend experimenting with heavy stuffing to theoretically lower it down to as low as 0.7 Qtc and for folks like me that are willing to potentially trade some mid-bass ?slam? for a ?heart attack fast?/critically damped 0.5 Qtc [higher F3], experimenting with a ~aperiodic alignment [heavily stuffed/vented] may be worth the effort.

Note that the latter theoretically has much better power handling down below driver Fs, so the best option overall if pipe organ symphonies and/or movie LFE playback is desired.

GM