Hi MAC,
Yes, I have the slide on the SD2 all the way up (to the right).

TASCAM has a round knob for MON MIX; At one end it says Input, and the other "Computer" (please see image URL)
http://www.tascam.com/i-3818-146-64-1-7CB8A7E4.jpg

I guess if all the sound is coming from the Inputs (midi from the SD2, or inputs from the Mixer), then the knob should be turned all the way Counter-clockwise. If the sound is coming from the PC (i.e. Realtracks/RealDrums or other audio files mp3 etc...) then I guess the knob should be turned all the way Clockwise to Computer). In cases where I have both (i.e BIAB midi tracks + Realtracks) then I am guessing the knob should be in the center, but I am finding this is not giving the best results.

I also have the Line Out fader all the way UP and both knobs for Line Ins (L & R) turned fully counterclockwise pointing to Line (as I don't have anything plugged in the MIC inputs). Does that make sense?

If I use the Mixer,
The Midi Out on TASCAM is plugged into SD2 MIDI IN
I am plugging the SD2 in tracks 5&6 with the MUTE button pressed down to send signal to ALT 3-4
I am plugging the Line Out from the TASCAM into Tracks 7&8
The ALT 3-4 Line Out from the Mixer into the TASCAM's Line IN (L&R)
The PC Speakers are now plugged into the Control Room Out from the Mixer
I have both "Source" buttons (ALT3-4 and Main Mix) pushed down and the volume pod next to them on 12:00 position (? is this OK?)
The fader for track 5&6 is at 0db, and 7&8 is on 0db (is this OK?)
What's the best position for the sliders for the ALT3-4 and MAIN MIX? 0db? higher or lower? Max?

And then, I have the separate volume control for the speakers...
Where's the best place ideally to adjust the volume?
Here's a URL with images of the Mixer...
http://www.behringer.com/EN/Products/1204FX.aspx

Another quick question: the Mixer Tracks also have a Level button with a choice for + 4 or -10db... What's the best choice for the SD2 & the TASCAM?

Thanks in Advance

2008 Gibson ES-339 Custom & 2010 Les Paul Traditional. Fender SCXD Amp
BIAB/RB 2015 UltraPAK + Ketron SD2
Windows 7/64 Intel i5 3550 PC, Tascam USB144MKII
Behringer X1204 Mixer
Bose Companion 3 and/or L1 II w/T1 + SM58 MIC