Old records.

@dweller ​​​​@pindac 

I do use a cleaning machine.  Perhaps not the best.  I believe the records have been damaged through the years from old and worn out styli.

Just use some audio intelligent solutions one step No. 6 and a vacuum record cleaner.  With 3000 records, I would think even with a bad stylus rotating through records would take quite some time for a bad stylus to do terrible damage.

Or, send a few of the worst ones to a record cleaning service and see if that makes a difference.  

The majority of my records are old. Those that were in the house when I was a kid were treated pretty dreadfully, being played on an old Garrard radiogram and then a Philips portable player. Stylus changes only happened when the thing would no longer play! And yet, right from my first RCM I found they could sound surprisingly good, and with better RCMs the improvement continues. I can't cure a scratch, sadly, but if a record isn't scratched it will usually be as near silent as makes no difference. The fact you say your new records sound great must give you some insight as to what's happening, no?

You may be right that your old disks were mistreated far more badly than mine, and they are damaged. The only way to know is to get a couple cleaned as well as possible and see if they improve. If your RCM is "perhaps not the best," have you the possibility of getting a couple cleaned by someone else for a test?

@rvpiano If it’s any consolation, my experience is much the same as yours. I have many old albums which have been meticulously cared for over 50 years…most of which simply sound flat and thin. Recording quality for most popular consumer vinyl was relatively poor especially for pop/rock music. 
 

I’ve reached a point where I only buy new vinyl for that reason. And it’s still a gamble. You can clean those old records all you like. It won’t change the recording quality.