gero,
There is no specification that will help you to best match your present speakers with a centre or surround.
You really need to hear the speakers together or know of someone (several ppl) who might have heard them before and can give you an opinion.
I recommend that you post more info about the brand and model of your present speakers and if you have questions about this subject, often referred to as 'timbre matching', then send an email or private message to Alan Lofft at Axiom.
He has heard more speakers than most animals on earth and will be very helpful in answering your question (assuming someone here on the forums cannot).
