speaker cable and interconnect different brands?

If you look at the systems that are posted at Audiogon, relatively few use the same brand name throughout the chain (if memory serves). I think that says a lot, considering the obsessive nature of audiophilia (is that a word?). Experiment and see. Naturally, manufacturers believe strongly in their products and that the "consistent" sound they offer is the best way to go throughout -- sometimes that holds true. Sometimes interconnects are one brand and speaker cable is another. Sometimes nothing is the same. Much depends on your specific equipment, acoustics and ear. There are companies (e.g., The Cable Company, I believe) that lets you try cables to see which work best. Usually that's set up so you lose money unless you buy somethng from them, but at least you are buying what you like best. Good luck (BTW, MIT speaker cables were a must for my last system so I hear you).

The brand is just what they stamp on the outside. Doesn't matter a wit. Only the performance matters. (And it's quite possible that different brands buy their wire from the same source.)

It's useful to bear in mind that only a handful of cable manufacturers actually make their own wire, or even assemble their own cable products. Most of the wire is purchased from a small number of suppliers, and many of the cables are now assembled in China or SE Asia. (I realize that some companies, such as Alpha-Core, Kimber, and others, manufacture their products in the US.) Hence, there seems little reason to worry about "mixing and matching" cables with different brands. The sole issue, it seems to me, is performance -- as others above have noted.

Having said that, if you find a brand that seems to perform particularly well in your system -- maybe due to some fortuitous combination of electrical characteristics that have a synergy with your electrical components and speakers -- you might want to stick with one brand. I have recently spent quite a bit of time playing with speaker cables and interconnects, and found that Alpha-Core Goertz speaker cables (MI2 copper) and interconnects (TQ2) work better in my system that any other brand I have tried, so I decided to stick with this brand for most of the important connections.

I think the key is finding cables which work well together, because using cables with similar but not identical characteristics together may give you a better sound than the same type all through the system. I use NBS Monitor 0 digital and speaker cables and Purist Dominus interconnects. The Dominus has a wonderful rich midrange and clear highs, but while full in the bass is less than perfectly clear there. The NBS cables have extraordinary clarity and detail in the lower bass and upper treble, and a fine midrange, but not as good as the Purist. The real key, however, is that they have the same general type of sound--full and rich. I might experiment, for example, with a Kharma cable, which is also similar, but never with XLO or Nordost, which sound marvelous in some systems but would never work in mine as it is now.

You are already on the right path by trying different brands. You do not need to match all same brand or design, just go for what sounds good. If that happens to be MIT, great. If you like Nordost or Purist, Cardas or JPS that is fine too.