Turntable sounds so much better when...

All you guys have been incredibly helpfull to me with understanding the reasoning why...And to think i was unsure of posting for fear of being rejected i guess. Thanks All.. Please at least try this if you have a removable headshell..and let us know..Richard

Sounds like when you loosen the head arm connection you are cutting it off from a direct connection of the negative, stifling, resonance that takes place in the arm itself.  The cartridge sends micro vibrations down the arm and that arm when locked tight fights how the cartridge wants to vibrate. The principle is like the old Victrolas that used to amplify the needle vibrations  into a big horn.  So.. You are decoupling the arm (to a degree) that has resonances that fight the cartridge from flowing optimally.

Sounds similar to the "not too tight" motto for the tonearm armboard nut on the Rega RB300/250, if over tightened the sound became deadened. Another thing might be the MC cartridge sending vibration back down the tonearm (I think more than an MM cart?)

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!