Hey fellow string obsessives, do you guys know of some flats that feel like double bass strings under the fingers? Obviously not gut strings...anyway normally I would research using The Internet™ but since this is a feel thing...it's up to you guys. I can get tone out of the amp and such so I'm just concerned with feel.
The experience will be several steps removed.
Electric bass flatwounds may be, on the average, lower-tension, but they have solid cores, which feature is now rare in double-bass strings, where it's been all but phased out in favour of rope-core strings, which are waaay more flexible.
Also, on an upright, they're typically plucked in a position that is proportionally similar to that of a P-bass pickup, but which is a lot farther from the bridge than the latter in absolute terms, hence even more forgiving to the plucking hand.
In short, heavy electric sets will give you uneven tension (a 075 D string might even have *more* tension than what you use on your doghouse; as for a 110 low E, you'll find it a lot less tight-feeling than it's often made out to be, and not quite in upright territory), and significant stiffness.
In order for an electric to feel like an upright, it should wear high-tension but flexible strings, and closer to balanced tension than what is usually offered.
If you're serious on this, either
1) look into the sole ropecore electric flatwound set available in the market, the Dogal Perfect Pitch flats Jonas Hellborg signature; specifically, get the 5-string set (JH1725S 34 G-B), toss the G, uptune the BEAD strings to EADG:
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or
2) get a 1/10 or 1/8 size double-bass string set. Make sure they're chrome-steel or nickel wound, so that the whole string, not just the core, is magnetically active.
Yes: both options are expensive. I can find 1/8 size sets for under 100 euros this side of the Pond: not sure about Stateside.
Whatever you end up getting, consider also moving your plucking position well into the fretboard area.