Well gas seems to be a variable we can rule out, and possibly the engine as the vibration source.
I don't think Canadian gas additives are much different than those in north east or mid west US.
In reading posts from other Canadians, ot appears we're not getting many vibrations at all.
My guess on the tires is we don't get them hot enough to flat spot, and the 1500 rpm vibration may have more to do with transmission than engine since our traffic conditions probably have us driving at variable speeds most of the time.

 

I think you should also consider the exhaust system. This is related to the low RPM shift points some think makes the sensation worse. The 2004 MDX chat rooms are filled w/a "resonance" sensation that Acura is said to acknowledge is a byproduct of the newfangled exhaust system used to eek out some extra HP. Something mechanical happens at some RPM change that (in non-tech speek) opens something up and changes the dynamics. But at a certain, relatively low RPM, the system is in transition and creates resonance sensed as vibration, hum, etc. I doubt the TL's exhaust system is markedly different. It may not be the only cause, but it is likely part of the sensation some are feeling.

 

Since were on the issue of vibrators.... Which kind do your girlfriends enjoy the most?????