Thank you for this.
Based on what you say I'm thinking that in view of the few M4A files I want to import and having had problems with stability in the past, the best thing to do is leave well alone.
What I've been doing is converting the tracks in Goldwave using 'what-u-hear' and then importing to PT.
This gives rise to another question that I'm sure you will be able to answer - can you record 'what-u-hear' directly into PT?
Many thanks again for your help.