Matelot B'stard,
You are on to our trick!! Actually, I agree that if one vibration is high enough, iut is difficult to judge if another frequency is behaving itself. Helo test pilots can get good enough to arrive within .1 IPS of a vibration amplitude without using the vibe gear. I use the physiological cues from my body to tell, (teeth slight chater at .5 IPS, etc.) On one test flight, I do recall having to fly with horizon reference because nothing in the cockpit was standing still enough to be read! In one experimental aircraft, the vibration was so bad my legs fell asleep, the blood couldn't figure out which way to flow, at the capillary level, I think.
Seriously, most S-76's can get quite smooth, especially in 1 per rev (1R). If not, there might be some tweeking needed on the blade. Email me for more info if you'd like.