Ok. After a full day of troubleshooting here is what I've learned.

If I change the transport settings to either ECO Slow CPU and leave the key lock range to always Fast CPU I get no pops and can drop to 7ms latency with the firebox.

Also, if I change the transport settings to HiQ Fast CPU and the key lock range to +/- 30% Slow CPU the same is true. I can drop to 7ms latency with no more pops.

Are there any advantages to one way or the other?

Sorry to keep posting myself, I am taking you through the steps I have taken and letting you know the effects of the changes. I seem to be a lot closer now.
Any thoughts?