Originally Posted by Heinrich S
My room is not nearly flat, but I'm trying to get it within 6-10 dB. I've been told that 0 to 3 db variances should be considered very good, 3-6 db acceptable.
The lowest note on an electric guitar is 40Hz,
Not a global truth. Electric guitars can be tuned by the user to suit, and generally 5 string basses go significantly deeper. I think it is reasonable to set the desired bass extension point for a music-only system at 32 Hz.
however there are harmonics that go lower than the fundamental
By definition there is no such thing. If there is a tone that is lower than the fundamental then it is properly called a subharmonic. Musical instruments that generate these are few and far between. Most involve very large diaphragms. There are also electronic devices that perform the function of subharmonic generation, and they are more likely to be encountered.
and these bits and things like ambience, etc, lives in the lower bass regions, i.e. bass below 40Hz.
Ambience generally rolls off both the bass and treble.
Most systems miss this. Coupled with a decent subwoofer(s) and room treatments and you will be surprised how much content you are missing in this area when listening to your music.
The guidelines that I am working with is that a music system needs to extend down to no higher than 32 Hz, and a HT system needs to extend down to no higher than 20 Hz. There are a goodly number of subs that can be clean and loud down to 32 Hz, but 20 Hz is far more challenging.
Often more than one sub is required to get bass that is evenly enough distributed across the preferred listening location. For example I have a friend with a massive IB subwoofer (4 x 18" driven by 4x 2500 watt amplifiers) and it turns out that his room has a natural null at 60 Hz right where his preferred sitting location is.