Just bumping this back up - I had an OMG THE NOISE experience with the Flexcore myself, and wanted to comment that in my case, at least, it was user error.
Yes, all other things being equal, things were 'noisier' with the Flexcore than with existing Bart NTMB in my bass, without changing any part of the gain structure in my gear. However, having recently gone through and tweaked gains and volumes and stuff to suit the Flexcore better, things are now no noisier than with the NTMB.
Both my overall gain structure and the master volume of my amp, BTW, went down to achieve the same perceived loudness during band practice with the same, very quiet, noise level.
The result, after these tweaks? Same perceived loudness (to my ears) as the previous preamp, with a much, more better sounding EQ section and a significantly less veiled sounds high end.