It sounds far worse without dust cover

I'm new to Vinyl world. I usually listen to records with closed dust cover & get generally more palpable & more coherent ambience than CD. Yesterday, I took off the dust cover & found all the palpable images were gone. Is that because the turntable is 2 feet behind the left speaker? Or , is that because of low-mass tonearm & high-compliance catridge?

Thanks in advance.
Sounds like an airborne vibration problem to me. When you lift the cover, sound waves are allowed to reach the arm/cart more directly. Most tables sound better with the lid off. Try changing the location of the table.