So I was bored last night and decided to compare 2 of my Jazz basses, sound wise. They are both Alder with maple/rosewood necks, and standard bridges. Here are the differences: Bass 1: Fender 60's Custom Shop pickups & TI Jazz flatwound strings Bass 2: DiMarzio Model J DP123 pickups & Fender 7150M nickel roundwound strings I expected some differences, obviously. But they sound surprisingly close to my ears. Strangely, Bass 2 sounds warmer and more mellow than Bass 1 with the flats! Maybe it's the pickups? Here is the soundclip. I play some random phrases. Bass 1 goes first, then Bass 2. http://www.danatkinson.net/bass1bass2.mp3
Those round wounds are dead. No zing left in them. If you play a string long enough if will sound flat.
Oh yeah. One of the key things to me in doing a comparison is having the same brand, type and age of string on both basses. If you can afford it, putting the same string type/brand on both basses and putting about a gig worth of time on each one (4 hours of playing or so) will get the string to the point of sounding 'how it does' through its 2-6 week life for roundwounds (depending how fussy you are about treble response), and will at least take the 'new string zing' off of flatwounds. IMO.