Electronic is better. Especially for subs. Passive crossovers reside after amplification and waste percentages of the amps power. If we're talking about hundreds of watts, you could end up loosing a lot and generating a lot of heat. Also, electronic crossovers are phase correct. So yes, just go with the LP filter on your amp.
but the lp on his amp is passive. the biggest problem, in addition to power loss, is that the vast majority of passives are 6-12db/octave, which still leaves quite a bit of room for the higher frequencies to sneak into the subs. that's a relatively bad thing. my subs cross at 63 hz with a 30db/octave slope. so i can assure that virtually no hz above the cutoff get in. although some high quality amps do offer 18 or 24 db/octave, an electronic will offer more room for tuning your other components (in capable hands).
"mikechec9 where did you get a a crossover with a 30db per octave slope?" it's a alpine pxa-h701. it's capable of 6, 12, 18, 24, and 30 db/octave. it's a monster unit. literally perfect. the only minor ill is the pause between saving and accessing settings (like 3 seconds). i have pics and a description of it on my cardomain <-----(underneath my username).