I wanted to add one last thing... as an additional test we clocked the MOTU interfaces with an external clock (Big Ben and a Lucid Gen X 192) and the results were cleaner Bass and Drum tracks with more punch.
It is a long known fact that the older MOTU interfaces had cheaper clock crystals...(Texas Instruments if I remember) even though the converters were sweet. There were companies (like Black Lion Audio) that would modify (replace the clocks) on the MOTU interfaces for a fee... We tried that route also, but the best results were simply to sync the MOTUs with an external high quality clock via BNC connector.
The best way to describe why an external clock would make the MOTUs track better is the following... If I have 44 thousand soldiers marching in step, the sound of all the boots hitting the floor at exactly the same time as opposed to 44 thousand drunk ass soldiers marching would be very noticeable. The high quality external audio clock ensures that samples are precisely synchronized and this produces to audio tracks with no "digital smearing" (as when cheaper clocking crystal sources are used).... think of comparing a letter written with a sharpie as compared to a $100 pen.
Be careful, the search for the best audio interface can lead to madness...lol...the older MOTU interfaces sound great for the money with no modifications needed... as pro results always start with the audio source and levels, so don't waste as much time testing things (as me and a few friends have) and just make music....unless you actually have as much fun with the technical side as the music....this has always been my weakness/strength.