sub pre outs on hu vs other

I'm going to be installing some old equipment from a past car into my current car. In my old car, I had an Alpine HU, a USX4085 4 channel amp for the front/back speakers, and a JBL 600.1 mono for a single Infinity 12" Sub. In my current ride, I purchased a new pioneer FHP8000BT HU. In the OLD setup, I ran the front/back pre outs from the HU to the 4 channel, then from that 4 channel amp, I ran a set of RCAs from the output of that 4 channel to the mono amp in. (low pass filter I think). In the old setup it all worked out great. Now that I'm setting this up in the currrent car, I'm wondering WHY I can't use the SUB outputs from the HU to the mono amp directly, instead of using the output of the 4 channel with the low pass filter. Anyone know if there are pros/cons to either method? THANKS ALL!Mg

If your radio has a dedicated sub out, it probably has adjustments in your radio to change the crossover point and the amplitude.   Doing it this way will allow you to adjust the sound and the output level of the sub from the radio.

Thanks! I wonder why I didn't have it set that way previously, unless I didn't have the outputs on my Alpine? Anyway, that makes sense to me! Thanks again