Vintage speakers Ñ could they be voiced for electro-caps?

I have been living for the last six months with a pair of Pioneer CS-655 speakers, one side of which has been modified, while the other has been left as stock. (Perhaps only Japanese readers will know these speakers. They remind me of the Yamaha NS 6XX series.)

For the modded side I built a piggyback board for the original board that mounts all the film cap substitutions that would not fit, as well as a bank of resistors to stand in for the gain pots I deleted. The system is hardwired as flat. The whole unit is now mounted externally. The cabinet has been rewired with better wire.

A similar treatment transformed some Tannoys I worked on, but these 3-way units do not promise such an astonishing transformation. Both the stock side and the modded speaker sound good. In fact, despite their mixed character they have supplanted my Monitor Golds which are parked nearby. The thing is this: I am not convinced that there is an improvement with the modded side despite all the gospel about film replacements being a no-brainer upgrade.

The modded side seems more detailed and a bit more open, or uncongested. But the stock side has something which I just can't describe. The has both intrigued and bugged me to the extent that so far I have not made the considerable effort to change the stock side to match the other. And so, I am wondering ... let me put it as a question:

QUESTION:
Could it be that some vintage speakers use electrolytic caps in such a way that are an integral part of the design? I am mainly thinking of control over PHASE SHIFT. Every capacitor manufacturer supplies data that applies to such engineering (donÕt they?). Could it be that film caps might alter the sound for the better in some ways, but upset the engineering?

I am very tempted to stick some fresh BP caps in the stock board and see what happens. Of course they will be different than the original BP's, but they will certainly behave closer to the original ones than the films in the other side. I can even see myself perhaps returning the modded board back to something close to the original.

Cheers Ñ Lorne