I installed many amps in vehicles over the years. (I'm not a pro, just something I like to do for myself or friends).
I could be way wrong on this, but I believe there is a specific reason that an amp is wired directly to the battery. Has something to with the power it draws.
Also, the larger the amp, the heavier the wire gauge you have to use to accommodate the wattage it's drawing. If you tried to wire it through the spod, you'd be running through lighter gauge wires I assume.
I'm sure a pro will stop in here and either back me up or tell me I'm way wrong. LOL