I put the largest LaBella flats on my L2, the S100M set, and they sound fabulous but bowed the neck very slightly, which worried me a bit. I know different Steinbergers react differently to string tension over the years and didn't want to take a chance, so I replaced them with lighter guage ones. A previous L2 I had had no problems.
Though they were very punchy sounding I didn't find them particularly upright sounding, due to the tremendous sustain and quick attack and evenness of the note typical of the graphite Steinies. I miss the woodiness of a P for example.
I'm happy to sell you them if you want to try them, they are like new and not broken in yet, still very bright. They do become more filled out and thumpy with some play, it's amazing tone for reggae, for example, and it is easy to control them with muting, but I'm not sure an upright sound is easily within reach with any strings on these basses. http://www.labella.com/category/prod...l-system-bass/
I tries Status flats and they were way too middy for my tastes,would work much better for rock IMO.
I looked on their site but didn't see the .109 guage I'm referring to, maybe they stopped making them. The nylon tapewounds listed above would no doubt give you the most upright like tone, though, if the sound of them on wooden basses is any indication.