Drew, 3. is a no and 4. was a "highly unlikely" that changed to a no with the results of your second test with an appropriate crossover for the M80s. 2. is more on target, although "devastated" is a bit strong. Good room EQ, such as Audyssey MultEQ, can help significantly. Re 1., Audyssey was doing the job that the settings in the 875 required. Specifically, for that model, as long as good response to 80Hz was measured, Onkyo required that the setting be "Full Range". Dr. Kyriakakis has pointed out that this is unrealistically high and suggested that the manufacturers use a 40Hz criterion(which Onkyo now does)or even better, delete any provision for "Full Range" or "Large" settings when a sub is in use.
The results from above 80Hz in your second test look pretty acceptable, considering how difficult room modes can be. So, change the setting from Full Range to the 80Hz crossover following running Audyssey.
