I notice that room effect has not really been mentioned except for a brief moving of loudspeakers "closer to the wall".
A sketch of your room would have been illuminating, Georgev. It is possible in a "typical" (whatever that might mean!) room to get as much as 20dB difference between the loudness of 40 - 60 Hz tones simply depending on where one stands (it is called standing waves). Adjusting of a loudspeaker(s) will move these positions and help somewhat, but never eliminate them. The position improves with notes of short duration, as low notes need some time to "build up", depending on the room characteristics (room Q). It will still exist, however. Also - and this is a disadvantage with stereo - two sources of low frequencies will always show cancellation where they are multiples of half-wavelengths of tones apart, and augmentation in between.
What to do? When you have good loudspeakers as mentioned, adding another or replacing might not help a lot. Try different positions, but be aware that there will always be standing waves i.e. nodes and anti-nodes, and that their comparative loudness' will shift with frequency.
PS: Sorry, on rereading I just noticed that you did move the speakers around. My remarks still apply, however. Hope you can get it satisfactory.
