In no particular order:
- there is nothing unfortunate in using roundwounds on a fretless. The champion of the instrument, Jaco Pastorius, used Rotosound Swing Bass 66 stainless-steel rounds, as did/have countless others, and that distinctive "mwah" tone comes from rounds, low action and a bridge pickup. The board may require some maintenance after a number of years. You could also use modified roundwounds
(groundwounds such as D'Addario Half-Rounds, GHS Brite Flats, Elites Groundwounds, Magma Flats;
pressurewounds by GHS or Ken Smith Compressors, Rotosound Solo Bass 55, S.I.T. Silencers)
which are gentler on boards and still vaguely in the roundwound tone ballpark
- a further alternative could be black nylon tapewounds (or, for a clearer appearance but also tone, La Bella "White Nylon" - it's actually transparent - in their three available flavours, to wit Gold=brass, Copper and regular White=steel); the typical tone of bass tapes, or guitar ones for that matter, is muted in the high-highs but, depending on brand, sometimes very favourably comparable to that of worn-in rounds, with a super-smooth feel and virtually no ill effect on the fingerboard
- having found out the traditional tuning for long-scale 6-string bass is BEADGC, do you plan to embrace it, or would you still prefer tuning to EADGBE, as some...longboard 6-string players actually do?
- if the latter, regardless of the naysaying you could indeed use that La Bella bass VI flatwound set, on one condition: whip out the measuring tape, measure your bass from ball-end to nut, and make sure the distance is no longer than 35.5". If it is, that set is absolutely an option (for E-e tuning). You may have to shave back some of the silk, and possibly also need to keep the string raised over the nut with a pencil for a week or so, until its initial stretching has the tapering point clear the nut slot entrance.
As to why bass strings should taper between nut and tuning post, see here:
https://www.talkbass.com/threads/bass-string-taper-on-basses-explain-please.1635166/
- if you're still interested in flats, by all means get flats - just be aware that their tone tends to be on the mellow side. On the other hands, there is a wide range of flavours you can get, depending on brand. You will have noticed by now that 6-string flatwound sets aren't cheap, nor are they plentiful. If you read up on the different broad tone groupings of flats (see for example here
https://www.talkbass.com/posts/27283101/) and find something that interests you, which YouTube video reviews confirm, you don't strictly have to limit yourself to 6-string packets: another option is getting a 5-string set, and supplementing it with a single high C from another brand. Alternatively, electric-guitar flatwound exist, some are available as singles from online stores (I'm a big fan or D'Addario guitar Chromes), and they're often long enough (their winding clears the nut) for bass use, if you thread each through washers or old bass ball ends in order for their small ball end not to slip through the bridge anchoring hole, and if you are prepared to wound it bare-cored to the tuning post if needed. Ditto if you go EADGBE: just get a 4-string set, add single B and high E
- as for the stainless steel tone and whether you'll find it offending to your ear, it would only apply to rounds (flatwound construction, which mellows tone, trumps any purportedly inherent trebly property of steel), and even of you opt for rounds you'll have to try and decide for yourself (you'll use the bass with a bass rig, as opposed to a guitar one, anyway). Not to mention the fact a fretless bass, with its lack of metal-to-metal contact at the board - replaced by the string's active end being between wood and flesh - is a lot mellower to start with.