Turntable sounds so much better when...

Hi i have a Pioneer pl-510-turntable and when i loosen the screw about a full turn, at the headshell/tonearm connection the sound is Amazingly better!! The Bass tighter then ever...Highs so crisp and Clear...soundstage much improved...im baffled...??!! I have a Denon DL-110 cartridge..PLEASE help me understand..Thank-you Richard
You should pursue all of the previously mentioned advise items. I have a Pioneer PL-530 and am the original owner. Bought it new in the 70's. There are numerous issues that can arise with tables of this vintage, such as speed variations, tonearm return point, strobe performance, etc. I'm not sure how similar your 510 is to my 530 but there are probably many common components that each share.
One advise thing I didn't see mentioned is possible cracking of the plastic tonearm base (if the 510 is of this construction). This is very common with Pioneer tables of this vintage.
First test is to grasp the tonearm between the head shell and pivot and push and pull toward and away from the pivot. Gently. Use no great force. Notice if you feel any play at all no matter how slight. If you do then inspect the tonearm base. There are a lot of Google results on how to fix this problem. Even if you do not detect any play in the arm I would still not rule out that problem.
Best of luck.

Too much torque and like others need to trust your ears!  How many cartridges have you tried?  I like Avid turntable but they are pricey but their entry level is damn good! 
I learned early in my audiophile times (ca. 1980) to really "fix these bolts and screws", to a degree - the idea in principle - just before deforming the material (which I never quite did).
I had to unlearn that - first initiated by a systematic listening session comparing different RCA connectors. I found that the lock-collar RCAs started to sound kind of stressed in an over-NFB-like quality, and shift the tonality up, tonality was screwed up. There was a certain, very light torque the RCA collars need for optimal sound.
From then on I started to check every screw (when doing a new setup, and when in the mood to compare). Whenever I compared screw torque the position with minimal torque sounded best (for me, to my ears, in my system), specially in cartridge and tonearm fixing. My brass cartridge screws on my ET2.5 arm start to brittle up the sound at about 30 degree more torque from a setting where the cartridge just stops slipping on the headshell from enough friction.This is for me my empirical systematic experience, ie. "my experienced truth".
To grasp theoretically what is happening is that while it would be desirable to have all combined parts of an arm eg. to move as one (which would mean more fixing) screw fixings between parts potentially creates stress. Just imagine a guitar string: To have it resonate as minimal as possible (and still have it lined up) you have to release the tension almost completely until practically slack. Increasing the string tension stores more and more potential energy into a resonant element, increasing the sustain (higher Q). And screws and metal parts are highly resonant.

If whatever experiment can't be repeated by others with the same

result than such experiment should be counted as worthless.