Closeup View
If you need more than simple summing, then you need the X.Sum.
The X.Sum is a 32 x 4 line mixer that can be used for external DAW summing or as a line mixer for synths, samplers, and effect returns. Any of the 16 stereo channels can be assigned to either the main stereo Mix output or the stereo Mix-B output.
Model: XSUM-03
MSRP: $ 2,585
X.Sum Owners Manual
The In's & Out's
Connect the X.Sum to virtually any line level source: balanced, unbalanced, transformer, or transformer-less. The  X.Sum has high headroom that will handle balanced signals up to +28dBu. All audio inputs and outputs are fully balanced and available on TRS connectors.

 Features
16 Stereo Line Inputs-
·
 Stereo level control.
·
 Pan/balance control.
·
 Mix assign switch that routes input
 channel to the main stereo mix or
 Mix-B stereo mix.
·
 Mono Switch.
·
 Left and right balanced TRS inputs.
Master Section-
·
 Master level control.
·
 Stereo/mono switch
·
 Monitor/Phone level provides
 adjustment to headphone jack and
 Mix-B output.
·
 Balanced TRS stereo mix output.
Expandable to 128 inputs.
Rear Closeup View
Looking a bit closer at the rear panel, you will notice a Multi-function I/O connector. This connection increases the functionailty of the X.Sum's inputs and outputs.
Breakout Box Details
Specifications
The X.Sum includes an external breakout box that adds connectors for the Mix-B outputs and "Pre" Mix-A outputs. The left/right "Pre" Mix-A outputs bypass the X.Sum's master section so you can use your own external preamps to add character and color to your mix.
Frequency Response (12 dB gain)
3Hz-54kHz (+0/-.5dB)
THD+n (Gain 12, Output +24dBu)
.0024%
Noise (22Hz-22kHz)
-102dBu (Residual output noise)
-87dBu (16 channels routed, faders off)
Maximum Input Level
+28dBu
Maximum Output Level
+28dBu
Input Impedance
30K Ohms (Balanced)
15K Ohms (Unbalanced
Output Impedance
60 Ohms
Dimensions (Mixer)
WxHxD=1.75" x 19" x 10.75"
(44mm x 483mm x 273mm)
Dimensions (Power Supply)
WxHxD=4.6" x 2.75" x 7.3"
(117mm x 70mm x 185mm)
Shipping Weight (Mixer & PS)
15 Lbs (6.8kg)
Read the X.Sum review in
Sound On Sound Magazine
 X.Sum Signal Flow Diagram 
 (347K PDF)
The Designer
The X.Sum is the work of Vince Poulos, the designer and owner of Speck Electronics.

Speck Electronics has been producing quality professional audio products for nearly 50 years.  Founded in 1973, Speck Electronics has consistently provided innovations in audio products for all segments of professional recording and touring. 

Other products include the via Fader Outboard Fader System, the EQF 810 equalizer, and MicPre 5.0 preamp.
X.Sum is an exclusive Trademark of Speck Electronics.