my idler is pretty modified for a Dual:
View attachment 422448
CNC parts, graphite center bearing, etc.
As it is, it matches the stock numbers for the belt drive 1249. It will put down W&F in the 0.04 range. The 1249 is also quite modified and gets better numbers than the Dual advertised for it, usually in teh .002-0.003 range. RPM is as stable as you can get for a non quartz locked unit. Both are in the 0.2% variance range for rpm.
The idler probably gets a little more time because of the Denon. It's a little "sharper" than the ADC in back to back comparo's. The ADC is a 1970's cart with a new elliptical stylus and the Denon is brand new, both done 2 years ago at roughly the same time.
But to be honest, you have to back to back them to hear the minute diff between them and even then, I can listen to either one and be totally satisfied.