[DanDan]

foxfyr, your point is well understood. I have earlier suggested using the term LF Decay and advocated such, to get away from the obvious misapplication of the RT60 term. I also suggest that useful Decay figures can be measured in less than perfectly diffuse spaces, by the use of judicious multiple measurement location. Not my idea, common practice.
To move forward I must say I like johnpm's Topt ideas very much.
Your confidence of non variance is simply at odds with my own and Andre's real life experience. I am afraid that divergence cannot be resolved by simple repetition.
May I pose a question here, which might shine a light?
Why does REW not present Decay figures below 90Hz?
Regarding the PA speaker, well obviously one would use a competent one. The suggestion was made in order to achieve high SPL, to get a decent S/N ratio. Obviously a bad anything is a bad idea. The Mackie I referred to is a fine transparent performer, with similarities in character to their excellent studio offering, which would also be a good choice IMHO
I reckon Ethan will agree on that one :-)
I would also guess an omni would be a good choice but I would certainly take big notice of the choice made by Bill Putnam and Bob O earlier, the SM57. Given the low costs perhaps several mics should be used, chosing the resulting varied tonal colours to suit the mix at hand?
DD