The old Emperor 400's that have a opening in the middle rear of the lid with the adjustable rate pull are worth looking for - the new ones I have seen have lacked quality control and have have noise issues. I don't know why Marineland has let things slip so much but they have, and I'm saying this after 3 months of working with them over filter issues this past year only to settle and make it work myself.
My situation - I bought two new 400's and they were noisy. I went thru the whole filter and was not able to eliminate it. My older 400's look almost the same but are quiet, and have been for 10+ years, even the older 400 witha chipped impeller is quiet.I get ahold of Marineland and they send 2 new filters, both 400's. The new ones do the same thing again, I send back the first set of 400's. To try to make up for it they then send me a Magnum 350 Pro as I site I wanted the 400's because of the bio-wheels. I get the Magnum and take it apart to inspect it before hooking it up. Before even taking the canister apart I look thru the clear bottom and see the yellow impeller has a fin broken off. When I take it apart the fin is clearly broken and missing, but there is no broken piece in the canister or box leaving me with the conculsion it was packed in factory like this and not caught in quality control. If something like this isn't caught I wonder what other issue Marineland is having (see the Stealth heater issue posts for another example). I get ahold of customer service again (same agent has been working with me this whole time). She is frustrated, I can't blame her, and she offers me two Penguin 350's for a final offer to settle it. Although less in capacity I accept it and they arrive. I have one running perfectly quiet all the time. The other I have to beat up on a weekly basis to keep it running quiet and often I unplug it during the day and plug it in only at night because I'm done dealing with it. I have one Penguin on each tank, but I also have an Aqueon (so quiet) on one tank and a Tetra (also quiet) on the other tank running all the time. I mainly wanted the bio-wheels and media capacity. Since doing all this, I have been lucky enough to acquire a few older Emperor 400's from a LFS that was closing for $15 a piece. I have seem some on Craigslist too. The quality of Marineland shines in the older design, these kick butt over the new design even with a chipped impeller on one of them they run quiet.
After doing some dissecting of the new model, I have found a few faults that Marineland could fix - lose the new impeller area inside and go back to the adjustab le one, go back to better grade magnets on the impeller, fix the motor in the base - the noise is coming from an imbalance in that system more than anything else, it's this system that spins the impeller.
Look for the older design and just make sure all the parts are there (you can find replacements but the quality of the replacement impeller makes me nervous. If it needs a lid, bio-wheel, or intake I wouldn't be concerned since those won't impact the quietness.
If I had to do it again - I would either have found old ones to begin with or would go with a different brand and use a sponge filter in the tank for the additional bio processing I wanted.
This link shows the old Emperor 280/400 breakdown including a exploded photo with the older lid and impeller system.
http://www.aquatichouse.com/FILTRATION%20MENU_files/emperor_manual.pdf