Yamaha TRB, used, should be a solid bass. A friend has played and toured with one for well over a decade. I think my friend's bass (TRB 1006J, I think) is 34" scale, though the Pattituci signature model is 35" scale (and likely over your price range, even used). Spacing is wide-ish, body maybe a bit on the big side compared to some instruments (my friend isn't a big dude, though, and manages it well).
Some of the higher-end Ibanez models are nice -- SR series are 34" scale with slim (16.5 mm) string spacing. Volos (I've not played one) are 33" spacing, while the "regular" BTBs are 35" with wider spacing. Another friend played one of the 35" BTB sixers for years before picking up an MTD.
Only had my hands on a USA Cirrus once, but it played and sounded great. 35" scale felt a hair big for me -- and I might have preferred a slightly longer top horn, ergonomics-wise (would have to have it in my hands again -- it was years ago), but it was solid, even, articulate, and fast. Distinctive tone.
There are all sorts of other basses you can find in that range (you mentioned $1500 and under), particularly used -- everything from small-luthier builds (used) to six-string Jazz-style basses.
I've spent some time on six-strings, but not a ton (don't currently own one -- maybe again someday). Still, I agree with the comment that scale length, string spacing, and other ergonomic considerations push themselves to the fore more with sixes than with fours or even fives.
If you don't know what you like, there, it's worthwhile to experiment. If you do know, then it would helpful to share. Even if you haven't played sixers much, it would be helpful to share what preferences/limitations you are aware of -- for instance, if you felt regular P-bass necks were big and awkward and felt more comfortable playing short and medium-scale four bangers, I probably wouldn't recommend one of the 35" scale BTBs. If you've played and enjoyed a few 35" scale fivers with 19mm spacing, I might. If you really don't know and are in the explore-everything phase, that's cool, too. Knowing might help folks to make more pertinent recommendations.
In any case, good luck.
EDIT: Tune basses were mentioned -- I borrowed a friend's made-in-Japan Tune six string for awhile (while he borrowed my amp). Very solid and articulate instrument. Ebony board, I remember. Some early Primus stuff was recorded with one, I believe.