I don't think it makes any significant difference in practice if you have a voltage amplifier because it has to damp one driver through the impedance of the other driver, series crossover components etc.
In the parallel config both drivers are effectively damped by the direct connection to the amplifier, or at least they are damped by the amplifier in series with the crossover, in parallel with the other driver which is still better than the series config.
tl;dr:
If your amplifier is a current amplifier then the series config is better damped. If your amplifier is a voltage amplifier the parallel config is better damped. If your drivers are perfectly matched it makes no difference!
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I finally got Soundeasy set up and I took some measurements. I don't know how much I trust the data under 1k, but above 1k it seemed pretty repeatable. This indicates pretty poor tweeter matching, but interestingly, the flatter response is the tweeter that was pushed in. Once I get a little more fluent with Soundeasy I'll start testing the individual drivers.
Looks like the characteristic midrange bump around 900Hz due to cabinet and driver response. Here's what I know about this series of Scan drivers. Presumably your 15W/8531K03 have a really small magnet for closed box.
The 900Hz notch that a lot of people use is not realistic in a MTM IMO. Takes impedance too low:
Ellam-XT
Ellam-FLEX
But Zaph's ZD5 has better impedance in the MT form, so might be adaptable:
http://www.zaphaudio.com/ZD5.html
Those time delay circuits can be swapped for third order with polarity reverse.
The magnets are the standard 8531 magnet, but as you can see from the pics, there are additional magnets added. Because of the high f3 and very small enclosure volume, I'm assuming these were added in phase with the original magnet to help lower the qts, allowing for an acceptable rolloff in a tiny box. If the magnets were added out of phase to raise the qts (such as a bucking magnet for magnetic shielding), it would dig deeper in a sealed enclosure, but that enclosure would need to be significantly larger. These cabinets are so full of bracing and ceramic plates that there's virtually no air volume. I'd estimate that there's only about .2 cubic feet for the two woofers to share.
But right now I'm much more concerned about the disparity between the tweeters than I am about the woofers.