Here's my thoughts.
That's a very tough choice given the fact that the open box 56TXi is still more expensive than the 72TX. However, the 56TXi has plenty of power supply and large capacitors to run this big ole E100 JBL's. Unfortunately, I can't comment on the other two as I don't own them.
However, if you are considering an HDTV with HDMI or DVI inputs, then it may be in your best interest to consider either the 72 or the 74. If you are big into music and have a collection of/are considering dvd-a or sacd, then you should definately consider the 56 and the 74 as they are both equipped with i.link(one wire multi channel surround as oppossed to 6 individual RCA added to the snake pit behind your entertainment center).
Like I said, I personally have the 56TXi and I have an HDTV with DVI, but I run everything throught component video and run my dvd player with a dvi cable directly to the tv so no worries there. Just something to consider.