So you want to replace a 8 Ohm nominal driver with 2 different 4 Ohm nominal drivers. What you seems to miss is the fact that a crossover is designed based on actual impedance, which is a function that varies with frequency, instead of a purely nominal value that is grossly derived from Re, which is the resistance of the voice coil. You need to compare the actual impedance with the derived impedance of the two new drivers (if in series simply multiply per 2 the impedance): if the curves overlap in a 2 octave range centered on the crossover point, then the actual crossover will work the same with the two new drivers. But this doesn't suffice as you also have to compare the raw unfiltered response of the new and old drivers. All in all without measurement equipment there is little if not zero chance of a success.
Ralf
Ralf
Re:'without measurement equipment there is little if not zero chance of a success" - that's a gross over-exaggeration; you can do a reasonable job working from manufacturer's spec sheets
WinSD can easily illustrate those kind of differences for you - just open 2 projects with the same driver, one sealed one vented.
You'll quickly realise why impedance figures like 4 Ohm and 8 Ohm are often referred to as "nominal", as the variations with frequency can be quite large.
As most amplifiers are primarily Voltage sources however, they usually take that in their stride. It's also why I recommended using the Input Voltage field rather than System Power in WinISD, as the latter is calculated on minimum impedance only, so not representative of an average across the whole operating bandwidth of the driver.