I've just been on the phone with Pinball Pro in the US, and to be fair Greg, the guy I spoke to, was very helpful. Basically he said that Metallica is an odd one as it is fairly bassy already, but that he would like to help me tune the speakers so they would work better with this particular machine.
He talked me through putting one of the connectors onto an alternative hidden lug on the speaker, which has improved the sound a bit, and said he would offer a full refund if I wasn't satisfied, although how that would work with postage back to the US I don't know.
He also recommended connecting a resistor inline with the connector to reduce the bass volume, and eventually we decided that a better solution would be to connect a variable resistor volume control to the bass unit, so that the balance between bass unit and box box speakers could be controlled better. I'll try this and see what the result is, but I was impressed that the company was keen to help.