I spoke with engineering for clarification on the questions you asked.
- Tones at very low frequencies have not been as much of an issue unless the engine is switching displacement. For the Audi, that will not matter until the new 4.0 TFSI is released with cylinder shutoff control. Another factor is the penetration of the tones above the broadband levels. Less penetration means the tones are less noticeable and multiple tones close to each other can actually be more pleasing to the ear. Our designs target the dominant tone around 100 - 130 Hz and usually increase the tones at lower frequencies (typical for resonators with tailpipes). If the tones are not significantly above the broadband levels, it's usually subjectively better.
- Yes, we could make similar plots for other speeds and the frequencies would be different. By choosing 65 or 70 mph for the marketing data, we are showing the speed that people usually cruise at on the highway. It may be worth noting we look at speeds in increments of 5 mph to make sure the sound is ok over a range of speeds and not just one as represented by the marketing data.
I hope that information helps! If you have any more questions, I'll do my best to answer!