Understanding + modelling passive filter (crossover) circuits: Component position and effect
Sure looks confusing to me too!

1) The diagram is drawn for differential signals, which is probably why it is confusing as it is different to a single input and ground. By differential I mean that the signal at - is the signal at + but phase inverted. These subcircuits probably fit into a larger circuit, but for modelling just the filters here you could just pick the + to be ground to find the transfer function.

2) To measure the output voltage you'll need a differential probe! This would be placed across the speaker terminals. If you don't have one you can use two probes, one from each speaker terminal to ground, and then use your oscilloscopes math function to calculate the difference. This will give your differential signal without a DC offset.

3) Ideally a speaker is not a polarised device by which I mean you can drive it either way. In fact this is integral to it's performance as you are putting AC into the speaker with ideally no DC, so the only change to swapping the polarity would be the phase inversion of the waveform produced, which can be important when working with multiple speakers.

EDIT:

Here is a quick model for you from the first circuit. By running a frequency domain simulation on this from 20 Hz - 20 kHz you can quickly see that it is a high pass filter with a cutoff frequency around 4 kHz. 

The Scan-Speak D2010 is spec'd at 8 ohms, but if you wanted to add some more accuracy to the model you could a series 0.08 mH inductance.







simulate this circuit &ndash; Schematic created using CircuitLab