request for help: 60-degree LF horn calc

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How would I get a first order acoustic rolloff at 300hz from a not-folded, conical, 60-degree, parallel-walled horn? My 300hz horn has such a LF rolloff, so the two together would afford a SET-friendly 1st/1st order, speaker-level XO.
The driver is a 15" Lambda Apollo M, with Fs 34.7hz, F3 80hz, Qts 0.33, Vas 312 liters, Mms 70g and Le 0.2mH (lots more stats, if needed). I could build as large as 4' square (3' deep with 12" opening for the driver).
Is there way to calculate the acoustic HF rolloff of such a horn with such a driver? I'd go directly to an 80hz edgarhorn, but I'd like a little more LF extension. I prefer the sound of my 2A3 SET to my solid state amp, even though its bandwidth and that of the speakers it will drive are slightly limited.
Above 300hz, I've got 103dB efficiency, with the option later of a boost to 108dB, so also, if making the driver opening smaller would increase efficiency (or help with the 300hz rolloff--since the horn would be longer), that would be a plus.
Any help would be much appreciated, as would pointings of the way to source material to study.
Many thanks.
keto

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Topic - request for help: 60-degree LF horn calc - keto 15:17:36 04/25/03 (5)

Re: request for help: 60-degree LF horn calc - Bill Fitzmaurice 11:18:01 04/26/03 (2)

Re: request for help: 60-degree LF horn calc - keto 11:39:41 04/26/03 (1)

Re: request for help: 60-degree LF horn calc - Bill Fitzmaurice 11:51:55 04/26/03 (0)

Re: request for help: 60-degree LF horn calc - keto 20:45:20 04/25/03 (0)

Some light reading - hulkss 19:54:00 04/25/03 (0)

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