Anywhere you put a decent polypropylene cap, it will be better than a good electrolytic, period. In old speakers the argument of esr is normally not as valid except maybe in some correction circuits. The old drivers and crossover parts have all drifted a bit. ESR is the resistance of the part itself and it is possible to effect the total resistance of the circuit. Normally, you are not looking at more than a 1/4 ohm or so.
If ESR is a concern, when replacing electrolytic with a MKP (Polypropylene) you can round down a bit, say you are replacing a 10 mfd electrolytiic, if you use a 9.7 or 9.8 mfd mkp, you should be fine. The larger the cap value the more that ESR could change. So in larger sizes, you may drop .5 mfd. For those that may not understand "why" a driver, may be called 8 ohm, but in reality it varies and may very well average more like 6 to 7 ohms, every part that you add to the crossover adds resistance, between caps, coils and resistors, you could very well end up with 8 ohms or more.... Good designers figure this in when designing crossovers. If you are really worried, i would recommend that anywhere there was an electrolytic to use a New, good quality Electrolytic and use a value a bit smaller, say 10 to 20 percent lower than the original and then add a good film & foil or mkp to bring the total micro farad rating up to the original part.... So for a 50 mfd electrolytic, use 40 to 45 mfd, electrolytic and add 5 to 10 mfd to match the original rating. This is still preferable to no film at all. This should make a difference, good luck, Tim