Have you tried different opamps? Is it socketed? The TL072A is a 8-pin dual opAMP right? You could solder in a socket and try any number of opamp chips in there to see if there's any changes to your liking.

This post has a ton of info on what to change on the V5:

http://www.tdpri.com/forum/amp-centr...ml#post1754695