Which cables cause issues for high bandwidth amps?

Cables having ultra-high capacitance are the ones that can cause the problem you experienced, if they are used without a properly configured Zobel Network. I believe Goertz can supply those networks, for use with their cables.

What "ultra-high" means, in terms of capacitance per foot, depends on the length of the cable, since the total capacitance of the cable is proportional to length. Following are what IMO are some rough ballpark guidelines for typical cable lengths, which are NOT based on any kind of precise analysis.

For typical lengths, I would start to become concerned if capacitance gets into the area of perhaps 600 to 1000 picofarads (pf) per foot or more, which equals 0.6 to 1 nanofarad (nf) per foot. Your MI-2's are 6000 pf/foot!!

For typical lengths I would not expect capacitances below perhaps 300 or 400 pf/foot to be a problem with nearly all amplifiers.

I would be increasingly cautious if the amplifier is one of those few having bandwidths in the megahertz region.

Regarding flat cables, yes, some of them have very high capacitance, but some do not. The Nordost's, for example, have extremely low capacitance, on the order of 10 pf/foot.

The great majority of speaker cables do not have ultra-high capacitance.

Regards,
-- Al

I want to see if I understand correctly that the spec that is the culprit in inducing these oscillation problems in amps is capacitance?

Yes.

And that as long as I keep that spec under 300-400 pf/foot, I am OK?

For typical lengths, say up to 12 feet or so, I would be confident of that. As I indicated, I am not basing that on any precise analysis, and a precise analysis would be dependent on the design of the particular amplifier. But I believe I am erring in the direction of being cautious, and so chances are that values that are somewhat higher than that would be ok too, at least in most cases.

Are there any other cable specifications that would ruin an amp?

No, I can't envision any other way in which a speaker cable could harm an amp, assuming of course that it does not have a short between the two conductors as the result of a defect.

Regards,
-- Al

Correction: I misread the Goertz datasheet. The capacitance of the MI-2 is 950 pf/ft, which is still very high of course.

Marakanetz, the high capacitance is a side effect of their approach to achieving their primary design goals, which were ultra-low inductance and ultra-low Characteristic Impedance.

Whether the ultra-low inductance is overkill or not, and whether the ultra-low characteristic impedance is likely to provide any benefit, are debatable and most likely system dependent, IMO.

Regards,
-- Al