Mission Argonaut, 1986

[runt]

Hi all, following on from my purchase of the quite wonderful Mission 770 Freedoms, a friend of mine has obtained the 4 ohm Argonaut from the same (eighties) era, and complains they are a tad light in the low frequency..!
We notice they have no vent port (unlike the 770F) any ideas on this?
Thanks in advance,
Paul/runt