I've never had a problem with the Meridain software setup, very straight forward and understandable, just remember to cycle the power on the processor after connecting the data lead to the pc. this has seamed necessary on some occasions for me.
As for the TAG vs M decision, when I was checking it out for myself a few years ago, I obviously fell for the M 568. At the time I was comparing the combinations of DVD players & processors. IMHO the TAG DVD just pipped the 596 but the sound was better from the Meridian.
One dealer chap had a TAG for himself and was plugging it to me, I visited the shop several months later and spoke to the same guy, he was asking if I wanted to buy his processor, he wanted to swap to a 568 Meridian!

598DP SDI, 568.25MM, Crystalio II 3800, 435 XDE (soon to be 50pf20?), NVA A80 x 8, AE2's, AE2 Pro, AE1's.