After some more searching and reading reviews, I visited a Swiss specialist based in Canberra, where I live, and asked what they charge for a service. He asked me WHY I wanted to get my Sub serviced: "Is it running poorly? Is it displaying any other signs of mechanical problems?" I told him that, as the watch is 6 years old, and consistently gaining 8 seconds per day (regardless of the position it's in), and as most service centres, authorised dealers, manufacturers, etc., recommend 5-7 year service intervals, I thought I should get it serviced. He advised that service centres generally recommend those intervals to "Keep money in the service centres -- to keep people spending". He said he'll check it on the Timographer and regulate it, which will cost $68. He'll keep it for a few days and see how it runs, and if it appears to need additional attention, we can go from there.
I've had several watchmakers give pretty much the same advice, so it sounds good to me. A damn sight cheaper, too.
I'll post an update a week or so after the regulation has been done.