The Westone MACH Lineup is devided into 3 Series.
Entry → For smaller Budget
Bass → IEM with an Reference Sound but with an (tastefully done) Bass Boost giving an more musical and “live” presentation especially on stage.
Reference → Faster and more accurate Bass giving an Studio Reference Sound for mixing/mastering and an analytical listening where everything is in perfect balance.
The Number in the Name corresponds to the driver count devided by 10 :P So the MACH 70 has 7 drivers and the MACH 80 has 8 drivers. The higher the number/driver count, the closer the IEM get to the reference target. So while the MACH 40 offers an very close match to an Studio Reference Monitor, due do its driver limitation, it can not get as close to this target as the 60 or even the 80. So the 70 is the best IEM for people wanting an bass boost and the 80 is the best IEM for people wanting an flat and accurate reference sound.
The difference between the Bass and the Reference Lineup is the Bass driver. The Bass Lineup uses one large BA driver for large bass. The Reference Lineup uses two smaller BA drivers for an faster and more accurate Bass presentation.
As shown here for the example of the MACH 70 and MACH 80
So you choose your Lineup and enjoy listening