Originally Posted by spike93
I have the [OEM under-seat sub] ...
Has anyone ever wired up a better amp to those or if its possible. Would it even produce better/more bass from that sub and speakers
People have used the OEM sub's T-harness to connect an aftermarket sub -- either another under-seat type, or one in the trunk. I've heard from more than one person who runs one in the trunk most of the time, and switches back to the OEM when he needs the trunk space or wants to shed the weight.
If you're talking about gutting the OEM sub to install a better amp (but keeping the OEM sub's speaker), don't bother. If you're going to that much trouble, get a better speaker too. Or look at one of the aftermarket under-seat subs. Here are a few:
Google around. You'll find reviews (even some from other Subaru owners).
As with the OEM sub, none of those are going to produce ultra-deep bass or rattle your windows. But several have a few added features over the OEM unit (e.g., higher power, remote volume control pod, RCA line-level inputs [line-level ins are good for when you switch to an aftermarket HU, or to connect to the line-level subwoofer output that some Subaru OEM HUs have]).
For anyone connecting to the speaker outs of an OEM HU, several of these do not have sound-activated amp power-on (like the OEM does), so you may need to add an extra "amp trigger" module to do that (there are a few out there).
Also watch out for the space limitation under your seat. You might try fabricating a cardboard box mock-up to check whether a particular sub will fit while sliding the seat front and back. Remember to allow at least a little room above the speaker for the sound to get out (and for the seat cushion to drop when a passenger sits in it
For pictures and more info on Subaru's sub and T-harness, see this post
(in my "Stock HU" thread here)).
Added: Some of those aftermarket subs suggest that you power them directly from the battery. But if their fuse is 10 Amps or less, you can probably get away with connecting to the OEM subwoofer T-harness, which has a 10 Amp fuse.
Added: A Subaru will not have the OEM subwoofer T-harness unless a subwoofer was installed at some point. They're not "pre-wired."