Subwoofer: Watts or Brand Name?

I've read numerous forums and reviews looking for a musical (fast, accurate) subwoofer to pair with my Harbeth P3ESRs (no movie use planned). When comparing my Harbeths to previously owned bookshelfs ($400-500) they sound tighter and less sloppy. I'm looking for the same in a subwoofer. I've owned plenty of $200-$300 subs that are a let down. I currently own an Energy S10.3 (10", 200W RMS, ported) sub. It performs very well as an invisible fill, but I want to hear "detail" in the bass. I want to hear the shape of the bass guitar string, not just that a bass guitar string was plucked. I realize that a JL Audio E110 or Rythmik F12 would do this, but I can't go more than say $500 used. Am I dreaming? I have considered a REL T5 or T5i, but I have trouble believing that replacing my 10", 200W, ported sub with a 8", 125W, sealed sub would be an improvement. Compactness isn't important. In the end I wonder if watts are the determining factor. A 10" 200W sub from a high-end brand name (REL, Rythmik, Vandersteen) versus a 10" 500W sub from a mid-level brand name (Energy, SVS, HSU, Klipsch, etc)... which do you think would perform better? The main reason I ask is that in my $500-used price range I don't know if an entry level sub from a brand name would be better than a upper range sub from a mid-level brand. Or to make life simpler, do you have any recommendations for a $500 sub (used or new) that is tight, detailed, and accurate? Maybe an 8" would accomplish this more than a 10". 

I've read over and over again that subs are expensive because they need (1) high watts and (2) thick walls/bracing. I wonder if I focus on the heaviest, highest watt subwoofer I can find from a mid-level brand...maybe it will outperform an entry level sub like the Rel T5 based purely on physics.