To use a USB DIRECT BOX XLR outputs
to feed the XLR input bus of my house mixer
with the USB output of my PositiveGrid SPARK amp,
(e.g., Peavey USB-P, On-Stage DB2150, Mackie MDB-USB, Radial USB-Pro, etc.)
I would have to fashion a dual USB type-B cable that is externally powered
to connect the USB Type-B output connector of my Spark
to the USB Type-B connector of the USB direct box?
We have XLR vacancies on our snake,
and wanting a clear stereo signal to our mixer is our motivation.
Checking any USB Type-B specification
we find that the USB Type-B connector
is normally to the slave unit that normally needs power;
whereas, the USB Type-A connector is to the host unit
that normally provides power.
Often the provided host power is not enough,
so we can find numerous mods on Youtube.
Using a powered USB hub is an option,
but I have boxes full of USB cables and USB power supplies to mod.
The intention is to make sure
the slave USB Type-B gets its 5 volts (if required),
and make sure the host connector power pin is disconnected
so as to not create problems having two power supplies
powering the 5 volt pin.
This mod is becoming more needed as many of us want to use
USB outputs of our amps and FX units into an analog chain.
The several USB Direct Boxes above fill this need,
albeit, with the proper cabling.
Cheers!

Pardon my asking, but why would you want to run a low quality USB audio output into your PA? Seems to me you're asking for trouble...

Of course, companies are generally willing to make any product that will sell enough to make a profit,. but why would you want to do that?

Is the SPARK's 24bit/48kHz USB audio inferior to
using its headphone output with a 1/8” Male Stereo plug to dual XLR adapter
to plug into the snake?
dk...
I would like to know for myself hence this post.
Many (but not all) user reviews of USB direct boxes
(e.g., Peavey USB-P, On-Stage DB2150, Mackie MDB-USB, Radial USB-Pro, etc.)
find the USB audio superior.
cheers!

It may not be inferior, but it might be, and it’s very unlikely to be superior.
All you would be doing is moving the DA conversion from your Spark to an external box. The Spark DA conversion is guaranteed to be seamless, the external box is not. If it has a different sample rate or bit depth to the Spark then you are going to degrade the quality.
I would guess the DA converter in the Spark is at least as good as the external DI.

It's an extra stage of conversion, isn't it? You should always try to eliminate superfluous conversion.