Upgrading opamps for better sound quality is very common request. The commonly used (and dirt cheap) JRC4558 may be popular for Tubescreamers but is not be ideal for circuits when you want the most transparent sound. Burr-Brown and other companies make excellent chips to retain sonic fidelity. Above is a photo of their surface mount chip which was used to upgrade the signal path in an EHX Holier Grail.
