The series 6 cabinet truly is near mint functionally and cosmetically alike. The previous owner did a great job preserving that marvel of audio engineering. It even came with the original cover and he kept the asking price pleasantly low: $400 (got it in February). As for your suggestion to swap the XLR connectors with Speakons; I actually use the 1/4" jack powered by a V-Type head - the signal is capital letters H.O.T. The slightest adjustment on the knobs is registered. Regardless of that, I did bring the matter to the attention of the local amp repair shop....the nature of the answer was along the lines of: don't fix what ain't broke.
I had gotten the 1818x series IV (just last week) thinking it comes with the same specs as the series 6 1818x (yes, rookie mistake). I can see myself either trading the 1818x series IV for a TE 1528 cab or TE 1524 cab alternatively, the TE 1248 cab. Whilst any of these cabs surface for sale more regularly in the UK or mainland Europe, it is rare hunting luck to see them for sale here in the US.
one more difference worthy mentioning: the inside walls of the 1818x series IV has the sound wool whilst the series 6 does not.