I don't have a TH600 but my understanding is it is the same as Denon D7000.
Oppo PM-1 is much easier to love.
PM-2 should love the same as PM-1
I was actually surprised to learn that the PM-2 headphone cups and drivers to be the same construction, if this is the case (I am not putting down the PM-1) it's the deal of the century from Oppo. I agree that the way the PM-1 model is built, it's hard not to love but if you are budget conscientious, having a similar sound signature is a bonus.