a better interconnect would be a good start too
the P3 is different than the other 2 in your upgrade possebilities list, I see that you don't have a TT, but do you have records? Vinyl is worth the effort, but it's getting expenive these days...

then, the sub, if you really think that you need more bass, the sub is the winner, but if you're happy with the amount of bass, don't do it, you're only going to disturb your neighbours...

the dvd player is also a good tought though...

Good luck,
Bert.