I've always been a roundwound player, mostly because 1: that's what all the basses I've ever bought have shipped with, and 2: because I like plenty of treble in my sound, whatever the bass.
With that said, one bass tone I've always loved and craved is Paul McCartney's late-years Beatles tone (from '66 onwards) - think "With A Little Help From My Friends", or "Something", that really full and rounded tone that sounds so beautiful and punches through the mix. I know there were a lot of factors behind that tone, including his switch to a Rickenbacker 4001 (at first), and, perhaps more significantly, changes to the recording techniques used as he chased a more Motown sound. I'm also aware that he probably used flats on his Rick. I know he also used a Jazz Bass in the later Beatles years, but I'm not sure what sort of strings he used on it, or which songs exactly he used it on, and searches have thrown up conflicting answers.
Anyway, I have a Squier Paranormal Jazz '54, which currently serves me primarily as a back-up gig bass, but also as a live bass on a single song my band plays that needs a really trebly tone that my Squier Rascal can't quite reach. I play with a pick, and achieve this tone by using the bridge pickup, tone at 100%, and EQing to cut the mids and boost the treble and bass. I do also sometimes use it for funky pick-played basslines too, in an Andy Rourke (The Smiths) vein, with a balanced EQ and both pickups at 100% tone.
I've thought about experimenting with flats, and this seems to me the ideal bass in my line-up to try them on, but there are two main questions I have before taking the plunge (and spending money on potentially two sets of strings if I don't like the results):
1: Would flats on a Jazz help me get close(r) to that post-'66 McCartney tone?
2: Would flats completely get rid of that absolute maximum J Bass treble, or just alter it a little?
Any tips or experiences from any J Bass flats users would be hugely appreciated!
1. Yes.
2. No, just alter it.
Have a listen to the trebly bass parts on songs like Glass Onion, and a few others on The White Album. A Jazz with flats, and all 3 controls up full gets you that sound. I don't know exactly which strings McCartney used, there has been much talk about it, and I'm not sure what the truth is - but I do know that with The Beatles, it was all flats. He didn't move to round wounds until much later, after the band split.
I've been down this road, I have a Beatles-themed Jazz (I posted about it a couple of months ago), which I intended to get a McCartney-like tone. Here's what I found:
1. For the most part, using neck pickup only, palm muted, tone control rolled off quite a bit, played with a soft pick, gets you a pretty good 1967-era sound (e.g. Getting Better, Fixing A Hole, Mr Kite, When I'm Sixty-Four, All You Need Is Love), and is also pretty decent for Something and Come Together, for instance.
2. With A Little Help From My Friends is a funny one, it's muted (presumably with the bridge mutes in the Rick 4001 rather than his palm), but a much more trebly sound than Getting Better, and those other Sgt Pepper era ones I mentioned. I can get something fairly close to this by turning the tone control up a bit more (but not full, necessarily). For this song in particular, using a soft pick instead of a hard one makes a big difference, I think.
3. All 3 controls on full, and no palm muting, nails that trebly White Album sound.
4. Wiring the pickups in series makes the sound fatter, and IMHO, better for the McCartney tone. This is how I have this bass wired. With this configuration, I mostly play it with both pickups on full, instead of soloing the neck pickup.
EDIT: one other thing, the pickups make a difference of course. Mine are wound to the spec of Seymour Duncan Antiquity, so vintage'60s spec basically. This definitely makes a difference - modern Jazz pickups might not get you quite the same sound. Particularly when you turn the tone up full.
Hope this helps.