First, the 3-to-1 rule has absolutely no application here. Seems to be a common misconception. It is an old wife-s rule that has its usage, but in a quiite different situation. It goes as a good approximation when you use two mics to record two DIFFERENT sources. If you keep each mic close to its source and far from other mics you get less spill and less phase problems between the mics. In contrast, in a stereo setup the two mics becomes part of one mic, a bit large though, and they are used to record one source, also a bit large.
Gunnar
Id be interested to hear where you got that info, or if it is your own personal theory, if youve done any wavetank experiements, etc... Seems as though every single person I have asked(on the professional side of the spectrum), every book I read(new stereo soundbook, recording theory, etc) have never said ANYTHING about different sources. p.. Surely working, professional engineers with experience wouldnt keep reissuing this advice in publications, verbally, etc..if it were just "a wives rule??"I use this advice all the time, and it serves me well, even though I do a lot of work with "stereo pairs as one mic"....and because of the simple existence of this thing we call "acoustics" is there such a thing as "same source" when using two mics???isnt even a stereo pair two different sources, technically speaking???I have told (and repeatedly so) that the 3/1 is universal and can apply to mono , two mics recording two different instruments, or stereo pairs recording the same instrument..
At any rate, I think why it wouldnt work has nothing to do with the source material , but moreso is a matter of logistics....a rock concert is limited in real estate...and is rather limiting because of sheer numbers of people in the area. I tend to trust the wisdom passed down through the ages from those who have established themselves.
What you could try as a first try is to EQ the bass a bit. Add some bass boost in post-production (for example using some PC-based program, Audacity is free).
EQing practices , unless for your own personal consumption arent reccomended.If you are going to spread the source it is better to leave as is , UNLESS you have a good monitoring enviroment and adequate tools. For your own listening, sure, EQ the heck out of it. If you do touch it,tell that you did.