But are all parameters equal Frederik?
I've got little technical knowledge on this matter but I would expect the 15 x 1 uF multiple cap to have a (even) lower inductance and resistance than the single 15 uF.
All caps suffer from some kind of resonance. The PIO types, and especially the ones created under high vacuum, I expect to have a relatively low resonance, and maybe their resonance will be somewhere in the audiospectrum where it affects the musical result to a lesser extent, but they will suffer from some kind of resonance as well. Maybe, maybe the multiple cap constructed with smaller value caps will have an even more benign resonance character than one big cap.
I think you're completely right about the necessity of having good soldering joints. Experienced this myself with the Scintilla xover (tortured by relatively high currents because of its < 1 ohm impedance) on several occasions. However with a single cap in a xover I expect that good soldering joints are at least as critical... A poor connection with one of the 1 uF caps and the other 14 uF caps connected properly will probably have a better result than a poor connection with a single 15 uF cap.
Kees