I took a peak under...
 

... the rear deck of my 2k3; on the right, there is what look like an upside-down amp, marked "Bose" : is it the processor/equalizer for the system? Also, just under the sub, there is a brownie-sized metallic box : is it the sub amp?

The black box is the amp. The brown thing under your sub might be the connector for it? The amp for the sub is built into the sub.

Stereodude

So, could someone tap the wires (line-level signal,I assume) bound to the Bose amps using rca connectors, then straight to aftermarket amps?

People have done this, but I was always suspect b/c I always seemed to find that different people had different opinions as to what wires went to what. There are adapters available that tap into these wires behind the head unit instead of at the amp. I would imagine that the wire colors would be the same at both locations and a knowledgeable person could jury-rig it to work just fine. I myself found it difficult to start splicing into wires with my $30k+ car. So I just replaced the whole Bose system from scratch and am very happy now.

Tony

That's an option i'm considering too. Thanks!

If that's the case, check out this thread. Hope it helps.

Tony

That's pertinent info, indeed. I'll print it!