The 505 series isn't as solidly built as the golden-age Dual idler driven machines (thinner deck plate and many had awful plastic plinths), but certainly with an Ortofon OM20, they could sound sweet enough, but with a certain scrappiness of tone that cheaper CD players have lacked for decades now.
Without knowing which 505 version you own, it's difficult to say more. if you have an AT95E, the tone will be a bit dull as the 95e rolls its treble away quite noticably. I understand there's a Shibata tipped version of the stylus available in the US, but know little more of it..
The thing is, the 505 won't damage any records you play and, if it fuels an interest, I'd try and grab a 1019 or a 1219/1229 idler machine, making sure the arm-height arrangement of the 1229 is working correctly. If you shop around, it may not cost you much to swap from one to another, as the 505 is newer and belt driven.
I deeply love certain old Duals and revere my 701. My current fixation with ancient Garrard autochangers is nothing to do with quality of sound, but the Duals I mentioned are true high fidelity grade and properly fettled can still be used as a main turntable. just avoid the later belt driven ones (1226, 1249 etc)..
Below, 1019
My 701 -