Not too sure about STTNG, but on most of the other Wms games of this vintage, the cab speaker is 4 ohms and is wired in SERIES with the backbox speakers... which would explain your loss of volume in the new speaker being 8 ohms.

And yes you're right the power amp ic on the sound board would just HATE a 2 ohm load!! Remove one of the sub wires to confirm if they are indeed wired in series, and see if the sound drops out completely.