@nonoise, I have also found little or no difference with my Isoacoustics Oreas. I have Gaia feet under my OB speakers but not sure about differences here. To fit them I bolted some 40mm angle iron to the sides which were drilled and tapped to take the feet. What this did was to help stabilise the speaker. I heard a slightly cleaner sound but don't know if it came from the extra stability or the feet or probably the combination.
@astolfor, you state you still have energy at 28.5 and 321Hz. This is possibly just some nodes found in all rooms which usually show many more than just 2. Your English is actually very good. There are some native English speakers on these forums who fail to use punctuation, correct grammar and have problems spelling.