When you are as old as me a thread started 4 years ago seems recent, so I'll resurrect this one since I had the exact same question as the OP and have an answer too...
I have 760M's (1954 Jamerson flats) on 4 out of 5 basses and have a custom set on order for my Mustang. Mustang currently has 760FM's (next heaviest set) and I had a set of FM's on my Spectorcore semi-hollow until last week. Obviously I like LaBellas, but I have experienced tone imbalance across the sets and it drives me crazy. Interestingly, this seems to vary dramatically by instrument and is a characteristic of the warmer flatwound strings like LaBella, GHS and Pyramid.
First the Spectorcore E string experience - this semi-hollow bass resonates and has an acoustic-electric bass tone to it (even unplugged). As a result, the mids really ring out on the A, D and G with both FM and M's. The E string on the 1954's was much more thuddy. I prefer the thud, but since the E is the outlier of the set I tried putting the E from the FM set back on. Happily the string size is almost the same. Probably a thinner core? Anyway, the mixed set of M's with an FM E string sounds very good together on this particular bass. I hate to mix strings but this one looks and feels similar, so I may stick with the setup. I once had Pyramids on this bass and loved the thumpy E string, but it was nothing like the tone of the other strings. I ordered custom sets of Pyramids with thicker A,D and G strings, but I kept snapping the heavier higher tension nickel strings which got to be too expensive (but the owner was a delight to work with).
My experience with other instruments is different...
On my Mustang, the FM's are thuddy on the E and A and twangy on the D and G. I am hoping the M's (or a mixed set) will correct this and will find out next week. I added an Audere preamp to this one which helps even things out when plugged in.
On my Stingray, the 760M G string was the outlier for a long time. Eventually it lost some of its zing and I also put a paper shim between the saddle and string to soften the tone a bit (that is a great and useful trick). The paper shim was ineffective on the Mustang.
On my M2500, the 760M set sounded great/even across all 5 strings from day one. The bass has humbuckers, but the tone differences I have observed can be heard unplugged too (in every case), so it has a lot to do with construction of the bass (unless the strings are inconsistent...which I don't think is the case).
On an Allen Woody that I no longer have, the GHS set was twangy on two of the strings and the LaBella set was twangy on one (or vice versa).
On a Steinberger 5 I no longer have, the 760FM's were thuddy on the B and E and brighter on the other 3 strings for about 2 years when they sort of evened out (just in time for me to get rid of the bass).
