I believe the version I have that works is their original v2.0 that I bought when it came out. Unfortunately, Synapse doesn't seem to provide legacy versions on their site. They had a number of interim releases before 2.501, including 2.1 and 2.22, but I don't have them so I must not have done those interim upgrades.
Interesting that 2.5 works for you. Maybe I'll try different options like switching from ASIO to something else, or turning off the 64bit processing engine in Sonar and see if that has any effect.
Interestingly, I can't see the 32bit version of DUNE 2.501 even though it was installed. Is this a Sonar "feature" that I can turn off in order to reveal and then try the 32bit version, if you happen to know? Maybe this is an issue with the 64bit version.
KEv