Well I could notice right away with every opamp swap. It's similar to changes on an EQ graph almost I would say. If you switched out the LM4562 and used an LT1369 instead you would notice that the mids and the drums sound more 'in your face' and sounds funner than the LM4562 but at the same time you would notice a loss of detail as though a veil has been placed in front of the sound.
The opa637 is better to me than the LM4562 and it sounds more like the prelude without an opamp but it colors the sound a bit. Makes the drums stand out more and is a bit to bright at the top end. Sounds more natural than the LM4562 but the LM4562 is not as bright and sound more balanced, (albeit very digital sounding).
I guess it all depends on how much effort you put into focusing on your music. Maybe if you listen to single albums repeatedly it would be more noticeable than if you just shuffle through random mp3s. Its a pretty big change though, I would say. I would rate it maybe 6-7/10 for difference in sound quality.
These are all very noticable things to me and I don't consider myself a nutso audiophile (no offense to anyone
) that needs to have special silver cables or separate power supplies feeding my amp or anything.
I was never one to believe that spending big money on separate source components would make a huge noticeable difference but after using the Prelude sans opamp, and having to use opa637's instead because of volume issues, I am interesting in buying a discrete output dac now when I can afford one.