I've heard the 73T and am having it shipped to me as I write. I A/B'd it with NAD 521BEE, which is the upgrade to the 521i. My impression was that the Arcam bass was comparable in amount but more detailed than the NAD. The sales person asserted that the NAD would have a somewhat weak bass from his experience with the 521i. I thought detail was better high and low, but the listening room itself was so odd that I couldn't confidently tell you about soundstaging. Treble of the Arcam was noticeably smoother and, well, prettier.
If I were you, I would listen to the 521BEE, not the 521i (which is discontinued anyway). The price of the BEE is $300 new, which is a great bargain.
Re: the Arcam 72, I haven't heard it. It uses a Burr-Brown DAC instead of the Wolfson that the Arcam 73 uses.
NAD models are 20 bit, Arcam are 24 bit.