I have STUDER A 80 2"/24 track (wide body) tape recorder calibrated on AGFA tapes...I record mostly rock music and I really love the sound of this machine, but I have never heard it with different brands of tape.
Also, it sounds way better on 15ips than faster...which does not seem quite logic to me...and I am sure there is a explanation for this.
Any opinions?

Try it out with different tapes. See what you find. There was a discussion on different tape types that you can look atthis.
It is good to calibrate and recalibrate to exercise the pots anyway.

15 ips might sound better because of the bass response. The bass bump is at a more attractive frequency. On older studer machines the frequency response is generally better (between the ideal 20-20K range) at 15 ips. The main drawback being noise.