Answering the question from the OP as seen in the comments:
The reason behind my question is that I feel like I am missing the
bass in the stereo output. Is it possible to tell from the amplifier
design whether the bass is separated/cut off from the L and R
The original link provided seems to indicate that this amplifier is based on the TI TPA3116 chip. Which you can find the technical documents here and the PDF here
Based on the datasheet, page 26, figure 37 "Schematic"; it seems that the components are wired to be in "master/slave" configuration to produce a 2.1 amplifier using 2 chips.
The bass is indeed seperated before amplification. This means that you do not have a bass in the 50+50 W stereo output. Without knowing the exact schematics of the actual board; I would not advise you to tamper with the electronics.
If you need a simple stereo amplifier, I would suggest that you source another board. Using the same site as the link indicates this shouldn't be hard nor costly.
I would not advise you to try to mix amplified signals to accomplish the same. While this may be accomplished using "reversed cross overs" this may be proven to be more costly than sourcing a stereo amplifier board.