Goertz supplies Zobels with EVERY set of speaker cables that they sell or supply to dealers. If you did not get the Zobel network, contact your dealer and raise hell. Unless you know for sure that your amplifier will not react to a high capacitance load, you should ALWAYS use the factory supplied Zobel network. I would not consider the use of the Zobel to be "optional" in 99% of the installations out there. The factory supplied Zobels are designed to stabilize the load that the amp sees starting at appr 160 KHz, so it will not affect the sonics of the system, only the stability of the amp.
As to hearing bright and grainy, that sounds like your amplifier crying out for help. Without the Zobel network and depending on the design of your individual amplifier, you may have what is effectively a dead short to the amplifier at VERY high frequencies. This can result in smearing and distortion of the signal in-band. Take the Goertz out of your system RIGHT NOW before you run into major problems OR install the Zobel networks pronto. If you do not do this, you are looking at blowing the outputs of your amplifier and potentially your speakers.
As to the sonic characteristics that you hear after the Zobel's have been put into the system, you are hearing what your system really sounds like. Goertz cables do not suffer from skin effect, limited bandwidth, high series resistance, etc... As such, it will lay the soul of your system out before you without mercy. At the same time, it will not introduce sonics of its' own because of the aforementioned electrical characteristics. If your system still sounds bright and grainy, time to start looking for other sources of the offensive noise that you can hear.
Out of curiosity, what length of MI-2 are you currently using ?
For the record, it is not a matter of an amp using large quantities of negative feedback that can cause instability to occur with highly capacitive cables. It has to do with how the feedback network is designed and the bandwidth of the amp.