I've replaced tweeters on several occasions and find that in some cases using a series resistor will increase the impedance and shift the crossover point slightly -more noticeably with higher order networks (such as the 3rd & 4th order filters most often used by KEF, or low impedance tweeters(4ohms).
I would recommend using an "L-Pad", which is two resistors (one in series, one in parallel) to maintain the correct tweeter impedance. here is a link to a page where there is a calculator for the correct values.
http://www.lalena.com/audio/calculator/lpad/
Try it both ways and see which you prefer.
FWIW, another consideration is that KEF may have used a Zobel/conjugate network to tame the fundamental impedance peak of the KEF tweeter. Unless the Vifa has the same Fs resonant frequency/impedance peak,there may be a mismatch(Just my 2 cents)
Good luck and good listening.
-chas
I would recommend using an "L-Pad", which is two resistors (one in series, one in parallel) to maintain the correct tweeter impedance. here is a link to a page where there is a calculator for the correct values.
http://www.lalena.com/audio/calculator/lpad/
Try it both ways and see which you prefer.
FWIW, another consideration is that KEF may have used a Zobel/conjugate network to tame the fundamental impedance peak of the KEF tweeter. Unless the Vifa has the same Fs resonant frequency/impedance peak,there may be a mismatch(Just my 2 cents)
Good luck and good listening.
-chas