I will do my best with this one.
As you know, the DV139 was the pinacle of Arcam's DVD line. It commanded a £550 premium over the DV137, due to some tasty extras such as Wolfson WM8740 24/192 dacs and two power supplies, to get the best out of both audio and video.
It also played full SACD in 5 channels.
The DV137, "audio-wise", was no great improvement over the previous DV79.
SACD is also only presented in stereo.
The DV137 is very good for audio, and one of the best in its price bracket but the DV139 is simply better in every way, and one of the best out there (for a DVD player).
It was worth the difference then, so if you can find it at reasonable money, it may make sense.
I only wish their new blu-ray worked as well, and was as high a quality - promising as it may be.