Just to add a little more knowledge, calling the UA 610 a DI is like calling Steve Vai an "average guitar player"
The 610 is a full-blown preamp, which happens to have a DI input(as many external preamps have). No matter what you feed IN to it, the output is a line level signal and that's too hot to feed into most mic preamps. Going into the Mbox via the TRS input is the proper way to use the 610. To use a standard DI box, then you would go in via the XLR input(but few DI boxes will have the fatness of the 610). As always, a quick read thru the manual for any DI or preamp will give you the proper info to get the best results