An aggregate device is a virtual combination of multiple devices: you do not need to make an aggregate device to use the K-Mix instead of your computer's onboard devices, so far as I know.

Ok..this is the weirdest bug i ever experienced:

my setup : stick =>Line 6 HX=>K-Mix=>amp
(Gr-55 , BigSky, Delay all connected also to the K-Mix)

while i was playing suddenly a certain note,G# on 2nd string 11th fret didn't come out-like it was dumped or something (12 string railboard)
my obvious thought was that it's setup thing..saddles..truss rod whatever.

the next day everything was ok so i left the setup issue as is

the day after same thing-same note -dumped.
trying to play this same note G# on the different string -again-no sound
fret above and fret below plays perfect !
also..and this is the weirdest issue : when bending this dumped note , sound suddenly coming out-as i bend the note sound slowly fade in.

i disconnected everything and connected the stick directly to different amp.
With this simple setup everything plays fine.

backed up my HX STOMP and did factory reset
connected the stick back to original setup and i got the same issue-
no G#.

as last resort and as hinted by this thread subject, i did a K-mix Firware update (although i already had the updated version)-and BANG everything back to normal.

anyone else experienced weird issues with the K-Mix ?

I had some wonky stuff going on with the K-Mix and had to perform a factory reset, but nothing like you observed. That sounds, well, just weird.

Did you try tuning it like a sitar? Sometimes that helps...

it seems that the k-mix is hard to tune...