Cascaded Filter Output Not Correct
The dB scale on the graph in LTSpice is relative to 1 V when measuring voltages or 1 A when measuring currents.

Your resistor values are 8 ohms, so 0.125 A AC is flowing through them in the pass-band of the associated filter

$$20\log_{10}{\frac{0.125\ {\rm A}}{1\ {\rm A}}} = -18.06\ {\rm dBA}$$

I'll bet if you put a marker on your graph, you'll find this is the value being reported.


  how could I keep the resistors at 8 Ohms (it is required for the project), but get the passband to be closer to 0dB?


There are lots of ways to do this:


Plot the voltage across R1 instead of the current through it (hat tip to Herve Grabas, in comments)
Plot I(R1)*8 instead of I(R1)
Set the magnitude of V1 to 8 V
Plot I(R1)/I(V1)
Replace V1 with a current source with magnitude 1 A.


The main issue is that LTSpice is sloppy. It's not really plotting a gain in dB. It's plotting a current or voltage in dBA or dBV.