Is there a difference between ΚΘ and ΧΘ?

Question

According to what I'd previously learned, aspirated stop clusters in Ancient Greek only had a single aspiration, at the end of the whole cluster. The reason for writing χθών "earth", φθόγγος "sound" instead of *κθών, *πθόγγος was convention more than anything else.

However, I'm now realizing that prefixes don't always follow this convention: ἔκθυμος "spirited", not *ἔχθυμος.

Was this just a spelling convention? Or was there actually a difference in pronunciation between κθ and χθ?

Answer 1

Here is Allen, Vox Graeca (15):

Allen and Sturtevant (Pronunciation of Greek and Latin) both argue, based on the rarity of misspellings of the type *κθών, that the first consonant in such clusters really was aspirated, i.e. this was not just a spelling convention.