Dash followed by comma (-,)

A question on punctuation:
(1) Sometimes, I write sentences which require a comma - like this one. (2) Sometimes - when I feel inclined to do so - I add an aside, enclosed by two dashes, to my sentence.
What happens to the comma when I do both. My logic says that the mere fact there is an aside, enclosed by two dashes, does not remove the need for the comma. According to my logic, the second dash should be followed by a comma. I have looked through a few books on the English language but haven't found anything which confirms or contradicts my way of thinking. In 'The Oxford Companion to the English Language' I found the following under the header 'Punctuation':
"The following extract shows commas, brackets, and dashes used in one fairly long sentence:
' The why and wherefore of the scorpion - how it had got on board and came to select his room rather than the pantry (which was a dark place and more what a scorpion would be partial to), and how on earth it managed to drown itself in the ink-well of his writing-desk - had exercised him infinitely.' (Joseph Conrad, The Secret Sharer)"
This sentence contains an aside within an aside. The outer one uses dashes, the inner one uses brackets. The closing bracket is followed by a comma, obviously because the comma is required with or without the aside. Would the same apply if the part 'which was a dark place ...' had been enclosed in dashes? (Obviously not in this sentence, because dashes are used for the outer aside.)
What do the experts say?