1) More headspace requires a finer grind. Finer grinds reduce mouthfeel.
Whoa there JayBeck. Now that is interesting - the bit about finer grinds reducing mouthfeel. I did not know that. Is this an informal observation or is it something everyone (except me) knows?
That would explain a lot. When I ran the E61 with boiler pressure preinfusion (usually about 30 sec), I found I had to grind considerably finer. I also noted a thinner mouth feel but I kept doing it because it made modern coffees drinkable (don't like sour). With the DE1, I am always using some sort of fancy preinfusion. Same reason. I will have to try the classic E61 profile and see if that changes the mouth feel from "a bit thin" to normal.
I seem to recall people saying that other long preinfusion/finer grind machines (like the Slayer) also tend to decrease mouth feel. I've also seen the opposite so I don't know what to believe about that.
CwD, a machine reading (viscosity) which defines subjective experience (mouth feel) is termed an operational definition. The temperature indication on your thermostat (the physical reading) is an operational definition of warmth (what you feel). Temperature works as an operational definition because we understand the relationship between temperature and warmth. We have no such understanding of the relationship between viscosity and mouth feel - and the studies which would give us that understanding would be very tedious. That is why the food industry uses tasting panels. This is all experimental design 101, and doesn't really matter unless you like that sort of thing. My opinion is that the easiest approach would be to just do the blind testing. If the effect is non-trivial, it doesn't take a lot of samples to achieve a valid outcome.