With the P15, having no mid control wasn't an omission. As
@nshuman mentioned, it is the preamp of the B15, thus no mid control. Additionally, in most situations I find a simple tweak of the lows or highs is sufficient for different venues. If you need more tweaking than that, a DI isn't going to solve your tonal woes. You need to look at the source and work from there (bass, preamp, pedals, strings, magic tone fingers).
Incidentally, the very first tube preamp I ever made was a Mid frequency
only DI. It was totally mojo city...cinemag inductor, Jensen DI, NOS tubes. But this ones mine!