worldcat,
I would say that the bigger question is... "Can you use a given tube amp with the Goertz cables." Assuming you are referring to the ribbon foil cables, then the question becomes one of the amp's output impedance at high frequencies. If the amp impedance runs in the somewhat higher range (above 2-ohms), then you might notice a bit of treble roll off. An 8-foot run of the ribbon Goertz cables could exhibit a fair amount of capacitance. I haven't measured them or done any calculations, but just guessing I suspect the capacitance could be as high as a nano-farad or more.
I don't have time to do the math, but let's just say that "in general" higher cable capacitance increases the likelihood of HF attenuation - especially when combined with amplifiers of higher output impedance. With most amps the output impedance rises with frequency, and due to their output transformer (among other reasons), tube amps more so. If you think about it, you always see power output specs rated at 1kHz. Few companies publish output power at 20kHz and that's because the output will usually be a LOT less than what it is at 1kHz. Of course the argument is that you don't need that kind of power at 20kHz because there's very little information in the music that high up, but nevertheless... advertizing the fact certainly doesn't help sell amplifiers!
Well, output power has a direct correlation with output impedance. All things being equal, the lower the output impedance, the more power the amp can deliver and vise-verse. So regardless of brand or design (sans some PWM designs like the NuForce Ref Series), most amps have far higher output impedance at high frequencies than they do at the low end. That being the case, you will experience significantly varying levels of performance when using high capacitance cables like the Goertz. I suppose that's probably why they've always represented a bit of controversy in the audiophile world.
I know Goertz offers some type of Zobel Network for use at the speaker end of the cable to "normalize" the capacitive reactance, but I suspect that only shifts the phase angle back to a resistive value. There's nothing you can do save adding a series RL network right at the amp output in order to actually RAISE the load impedance seen by the amplifier, and any practical sized "L" will waste amplifier power across the whole spectrum (not to mention wrecking the amp's Damping Factor). That being the case, if the amp in question does exhibit a higher output impedance at high frequencies, you *might* experience some HF treble loss using the Goertz cables - albeit most likely only a dB or two... barely noticeable, if at all.
OK, so what about the Magic Cubes? They present VERY LITTLE additional capacitance to the output of the amp - nowhere near as much as the Goertz cables. If the tube amp/Goertz combination sounds good without the Magic Cubes, then it will also be fine with them... and probably a bit better. At that point though I really don't recommend adding any of the filters that were offered with the MCs. With the Goertz cables you'll already have plenty of "filtering" action going on. I hope this helps.
Take care
-Bob
