Anyone ever opened MIT cables?

The "boxes" are not filters per-se... The inductors, caps, R's in the networks are, to my understanding, a Group Delay Equalizer. This accomplishes the time-alignment of various frequencies traveling down the cable, such that they arrive at the speaker's terminals simultaneously. MIT explains that this phase-correction may contribute to perceived louder-sounding (and quieter = less noisy) performance than with a non-aligned cable operating at the same power level. I only have their 3rd, or 4th-best cable down from the top. Still kinda pricey, but so worthwhile! GREAT product. Spectral even 'requires' the use of MIT with their gear. Certainly a respected manufacturer, not to mention their design guru Keith Johnson. Think he just might know something?

My understanding is that they affect impedance, and therefore use a parallel filter network.

No once again they're expensive tone controls. Spectral is known to be bright at times why do you think they recomend you use MIT cables? And CARL if I remember right you have a krell amp another company that is know to be bright. So it's no wonder you like MIT. I had there 770 wire and 330 plus series 3 stuff. It still sounded great but I didn't need to "fix" anything when I got my new amp and I could now hear exactly how much the MIT wire was changing the sound compared to non boxed cable. MIT wire intentionally warmed the sound at the expense of some imaging and detail loss and a little less bass. That was unacceptable for me. I didn't need it to do that nor wanted MIT for that. So good riddens to $5000 worth of tone controls. However if that's what somebody wants and needs in their system and they are happy then more power to them. MIT doesn't suck if you need them but they are still just entirely over priced tone controls. And they still beat a lot of cables out there in the right systems that need them.

Most cables are overpriced IMO. They are also system and taste dependent. I use both Homegrown Super Silver and HT Truthlink IC's (switch them out). They sound entirely different and I like them both at certain times and for certain music. All cables (that I have tried) alter the sound and are therefore a type of tone control. Otherwise all quality cables would sound alike - which they don't.

Actually, the 350 Shotgun EVO sounds exactly the same, either with my Krell or my tubed Rogue 88. It is not a tone control, but I agree that some MIT cables have approached that effect. So, NOPE, you can't just dismiss them out of hand, and you need to try other MIT cables, Kacz. I never said they all sounded the same, and I do feel that the new series are better than the old, especially the "Highend Series" (at least the ic's, I don't like the 750 speaker cables). I also currently own the inexpensive Terminator 2 and 3 speakers cables, and they ARE NOT tone controls either, any more so than the many other cables I've tried, or that I own.