I picked this up (used) as a backup recording interface.  Everything checks out except I can't figure out how to direct monitor (if its possible).  Only thing I found to do is enable echo on each track in Sonar.
 
 
 

Yes, it is.  Quite easily.  You need to use the Settings Panel.  A few minutes with the MIXER and OUTPUT SETTING tabs should get you going.

I see I can change Line out 1 and 2 to Master L and R, instead of computer.  That allows direct monitoring but the levels are goofy if trying to track with existing tracks.  The output from Sonar has to be dropped way down to be able to hear the input at the same time.  e.g. I plugged an SR16 drum machine into channel 1 (through a D.I. Box) and set the level to -4.  I had to drop the Sonar output level way down so I could hear the "direct" drum channel. 
 
Either I'm doing it wrong or it just works different than other interfaces (including my main unit an MOTU 896mk3 Hybrid and the Roland Octa-Capture I used have as backup unit).

Try this.
 
From the Output Setting tab, change Line Out 1 from Computer 1 to Master 1 and Line Out 2 from Computer 2 to Master 2. Note however, that this is still in PARALLEL (summed) with your computer audio output. You'll need to make sure nothing is being sent to the mains from your computer.
 

That is the way I set it. 

fireberd
I see I can change Line out 1 and 2 to Master L and R, instead of computer.  That allows direct monitoring but the levels are goofy if trying to track with existing tracks.  The output from Sonar has to be dropped way down to be able to hear the input at the same time.  e.g. I plugged an SR16 drum machine into channel 1 (through a D.I. Box) and set the level to -4.  I had to drop the Sonar output level way down so I could hear the "direct" drum channel. 
 
Either I'm doing it wrong or it just works different than other interfaces (including my main unit an MOTU 896mk3 Hybrid and the Roland Octa-Capture I used have as backup unit).

I assume you made the appropriate level adjustments via the MIXER tab?
And don't forget that Line 1/2 have a knob on the front panel.
 
There are gain adjustments for inputs 1 - 10.
 
Why are you using a mic IN via DI?  Just curious.
 
INs 11 - 16 (rear) are line inputs switchable -10/+4
 
IN 9 and 10 (front) are INST/LINE switchable with gain adjustments.

I am set up for  recording session tomorrow using the drum machine.  I have an XLR patch panel that is wired to my MOTU unit.  The output on the SR16 is 1/4" and I have a D.I. box to convert it to XLR.  I just moved the XLR cable to one channel on the Tascam unit.

I wish I could help more.  Does the Tascam DSP mixer view show decent levels from the SR16?
 
How are the outputs being combined for monitoring?

I got it going last evening at least close enough that I could use it if needed.  Direct monitoring for recording is not an issue, now that I have the Line Out 1 and 2 set to Master L and R.  I worked with the levels in Sonar for tracking and have it that I can work with it, if I need to track with a recorded project.  This will be used as my backup recording interface so after getting it configured, it may just sit in my rack and never get used.
 
Thanks/Jack
 

The 16x08 is pretty close to my older version the us1641, But I see what they removed from the front panel is the direct monitoring controls? Pity. So easy when there is a balance control like the older models. Now it's a Software mixer , just like my 6i6. Even after 40 years of running just about every brand of mixer on the planet, I've yet to figure out how the dang thing (Mix Control) works,,, I simply don't even touch it and use a hardware mixer for monitoring. 
I can see it would be beneficial if there where more than 4 musicians in the room but looks like a nightmare of button pushing and sub menues and then assigning outputs in Sonar sub mix busses. .
 
Looks like the Tascam mixer was designed by the same genius ;)
 
But you'll find they are solid build and mine is still doing a great job of live recordings. I even trust it enough to use as my output interface for my live gigs now. Tascam seems to be getting a solid handle on drivers and I'm looking at that UH 7000 as my next purchase. Hopefully it doesn't need a GUI mixer.