The two woofer/driver speakers are one-way speakers (ie no mids or tweets), which is pretty much the usual with a guitar amp anyhow, since the lows to mids are the usual frequences we worry about in the guitar amp world.
But... considering this IS also a 'bluetooth' speaker you can play your own music on, would be nice to add a couple two-way coaxial speakers, commonly found in the car-audio realm.... I honestly don't know if they are passing the full freq. spectrum to the AMP section anyhow, they may have a low-pass filter setup somewhere along the circuit to cut all mids and highs out...
The amp is stereo (2x 50W), so each 4 Ohm speaker is connected to one channel of the 2 channel amp. My guess is that these speakers are rated at 40W, which is why it appears the TPA31162 amp chip (PLIMIT pin 6) is setup with a value, to limit the max output power. (ie 40W max versus 50W full power of this chip).
*** UPDATE: ****
I did check out the PLIMIT SETUP, there is NO resistor divider setup, they had the footprints there if they wanted to drop the output power, but currently PLIMIT is tied directly to GVDD, so indeed this AMP is setup as full power, 50W x 2.
So really we will just have to test it out unless we can identify filters on the PCB....
The 'cutout' holes are 3.75" diameter, and the mounting area for the speaker 'flange' is around 4 1/8", a little more in the areas where the corners/screw holes are because of the odd shaped mounting flange.
Shouldn't be too hard to find some car audio 2-way coaxials that have a 3.75" (or smaller) cutout size, as long as the mounting area is larger than the 3.75" hole so we can mount them!