kalali, you should read up on the Schroeder Method to find out, among other things, that it is a "do at your own risk" technique. It is in the experimental stage in terms of finding out the parameters of its use.
It is recommended to use four identical ICs, but I believe that member jayctoy has tried combining ICs of different makes. You may wish to talk to him here as well. As far as improvements go, I presume that even when using mismatched pairs of ICs the Schroeder Method would provide potentially superior results over a single IC.
Also note that it does not take expensive ICs for it to work.