I am having exactly the same problem as dougal.
WHDLoad and kick34005.a500 with the A1200 works without a problem, but on the A600 I get invalid file error. I am using a registered version of WHDLoad 17.2 and the Kick34005.a500 comes from my Amiga Forever CD. The RTB and PAT files are from skick v3.46. The A600 has an ACA620 and 2MB Chip and has a 3.1 ROM installed. Will try the snoopdos although I haven't used it before.
Same problem and setup here, although I got the A604n instead of ACA620.