a summing amp is used to bring multiple sources together
it may seem simple but it has some special requirements that a simple line level unity gain amp doesn't

a simple situation of two line outputs joined to gether to make a simple mixer.
each amp sees the other as a load
simple solution is to feed the two sources through a resistor such that each output/amp is not overloaded (loaded DOWN)

as this gets larger and more and more are brought together ... that summing point seems to get near zero
virtual earth summing junction
the signals seem to get smaller
so
the following amp .. the summing amp ... needs to be low noise and high gain ... and headroom to burn
and a high group time delay ( don't ask )
and a phase shift under 90deg at the crossover point ( err definately don't ask)

they are just better amps than your average line level, EQ, buffer ... general purpose amp

some Mic-pre amps do find themselves used in this situation.
The Neve BA283 and API2520 and cousins are obvious old time popular units

A modern use for these things might be a PASSIVE summing unit
for analog mixdown from Digital Interfaces

might also be used to bring two stereo programs to one
....
stereo mix with a parallel compressed mix
mastering engineer trick

add the tube feel of the Tube Tech and you might have some of that MASTERING magic

all a bit too short and incomplete as an explanation
but I hope it gets the ball rolling

Kev, awesome explanation!

Would you use a summing amp for sub mixes, bus out certain groups like bass, kick, snare... and maybe even the lead vocals?

Example, to give all the center panned tracks better imaging?

Or... to use it for the sweet tracks in a song... kind of like adding a hook from a sound designers POV ... ear candy...

Or... are they more for the final mix ?

all of the above

current trend is to do sub mixes in both stereo and mono from the DAW and then sum these together in analog for a final mix

yes imaging is one of those qualities people are looking for

I guess the more inputs to the passive mixer there are
the more people tend to take out of the sub groubs

some units don't have a pan ... as these are hard to get trimmed equally across all channels
same goes for faders
...
here is why top end mixing desks are SO MUCH money ... quality parts

back to the point
some units have a switch for left, right and centre
and have a fader as a rotary switch with matched resistors so each channel tracks the same

The DAW can make a stereo synth or guitars or drums and the imagining is ok
then these are brought into a larger mix using the passive unit and then the centres like lead vocal
... kick and snare perhaps.

As for the sweet tracks
well people use whatever trick they can to make a musical part sit inside the mix but easily heard... without pushing it on top of the mix

some musical parts get transistors and some get tubes
at the time of recording
or later at the mixdown
:roll:
or a bit of both

it's all about colour
and then about imaging and openess and ... glue
all at the same time

difficult to put into words but when you hear it you will be chasing that feel for the rest of your recording life

Hey Kev...

"when you hear it you will be chasing that feel for the rest of your recording life"

beautifully put...

now thats a signature definitely worth using!

Summing Amp

Check out the Dangerous Music 2-bus. It's a fantastic analog summing bus for digital recording.