The archives go back as far as 2000. See
this page, for instance, and check out Hunter x Hunter at the very bottom: you get its ratings for the second half of 2000.
Now, it's very tricky to compare the two, given the vastly different situations: Shounen Jump was even more popular back then, and so was anime on TV as a whole (it's pretty obvious if you read the figures for the other shows on that page, today's figures pale in comparison). If you get into details, even the channel changed between the original TV anime and the remake, and the old timeslot was actually better, so it's only a given that the ratings are lower for the new series.
With that said, the new HxH is actually performing fairly well: it does better than all the current WSJ series but One Piece and Toriko (sure, it has a solid timeslot, but it isn't amazing either), which sometimes even means a rank in the top10. I don't think the producers are
too worried about the series getting stopped any time soon, if this is what you were wondering about.