Are your filters Polycontrast, or PolyMax?
The most recent Kodak filters are labelled as PolyMax, and they do provide a wider range of contrasts with variable contrast papers.
That being said, just about any set of filters will provide you with a wide range of contrasts. The grades won't match exactly from brand to brand, but they won't match fixed grade paper values either. As long as you work within a particular system, you can get good results.
With respect to whether below lens filters work as well as above condenser filters, I think the best answer is that if they are clean and in good condition, the below lens filters work very well. They also have the advantage, in my experience, of being easier to switch without causing enlarger movement and vibration, which is important for split grade printing.
If I were you, I'd try the set I have. If you see a good response with the contrast choices available to you, and your prints are sharp, then there is no need to change.
One further point though - all filters will fade over time, and exposure to bright light will accentuate that problem. If the filters you have are already faded, you could benefit from new ones.
Matt