Whoa...Tekunda, whoa! The velocity of propagation can be argued to be entirely irrelevant to a signal that is restricted to a maximum rise time LESS than a 20kHz. sine wave (at that!)...In fact it is virtually irrelevant to most RF applications.
As far as an adjustable "current rise time selector" someone will have to tell me just what that is in reality. Since the voltage and current are directly related and linked, I see no way to adjust the "current rise time" without altering the frequency response of the cable - ie. a filter.
By definition, ALL bass speakers are "slower" than tweeters, IF you mean that the time between a min and max excursion is longer, which it HAS to be because it is a lower frequency.
But if you are talking about acceleration, that's a different bird. It may be that some woofers are "faster" than some tweeters if you are talking about acceleration.
The *real* effect of speaker cables comes mostly as it effects the waveform produced by the amplifier, especially with reactive loads and/or amps that use a lot of feedback (which most do) and/or may not be totally stable into some loads or impedances.
If you do something via a cable that adds subjective "speed" or "impact" (etc.) I'd bet you can *see* it by looking at the effect the cable has on the output of the amp when you use a nice clean square wave as a test signal - in most cases you'll see the leading edge alter and the overshoot change. I'll give you odds on that one.
