Well, I finally picked up my MOTU 8pre today, and just got it hooked up. HOLY COW. I'm coming from an M-Audio Audiophile 192 with a Mackie 1202VLZ as the input, which sounded brilliant; and this is like an order of magnitude smoother. It just sounds, well, expensive. I had no idea; I really though my system sounded pristine. This is just amazing. The noise floor is 30dB lower too. I have a little ground loop somewhere, that ran about -50dB on the 192, and it's about -80dB on this. I presume that it was the Mackie, which I'd ground lifted to help it out already. Think I should ground lift the MOTU?
I don't recommend ever lifting the ground on equipment. If you have to do that, something is wrong, and you are making your rig significantly less safe. Instead of removing grounds, you really need to add grounds to one or more pieces of gear---whichever one has a poor ground in its power supply (or if you have any pieces of two prong gear, probably those... maybe wall-wart-powered gear also).
Fundamentally, a ground "loop" (I hate that term; it's not at all a good term) is caused by one device that generates noise on its ground plane and does not have a well-grounded power supply. Because this noise tries to take the shortest path to ground, it travels through the shield of your audio cables towards any other device whose chassis/signal ground is properly grounded, then goes to ground. When it passes through the shield of the cable, though, it is leaking a small amount of noise into the signal.
The way to solve this problem is to provide a better path from that noisy device's grounds to an actual earth ground (which can best be provided by the ground pin of an electrical power cord).
To track down the problem gear, take a three prong power cord and cut the socket end off so it's just a standard plug with a long wire attached. Cut the hot and neutral wires shorter and tape them up really well. Take the ground wire (be ABSOLUTELY sure you get the ground wire!) and touch it to one of the jack shields on each device until the hum goes away. You may find that more than one device improves things, in which case you need to ground more than one device....
The best thing to do from there is to make up a series of little audio extension cables—one each for every piece of gear you want to ground better. Each one should consist of a plug on one end, a jack on the other, and be wired just like a normal audio cable. In addition, at the end that plugs into the piece of gear, you should connect about a 10 AWG (or larger) copper wire to the ground pin on the plug.
Tie all of these grounds to the ground pin of that power cord/plug. Once you've done this, the noise should go through that heavy wire straight to the building ground instead of through the shield of your signal cable.