If the Search Forums would work I would give you the link to a lengthy thread on the S4 tips. They are cheap chrome steel, welded top & bottom to the exhaust pipe, which is apparently a poor grade stainless steel. At the exhaust place I went to, they were extremely reluctant to attempt to break the welds, for fear of damaging the valve on the rhs (on the left side the valve is before the muffler to actuate both pipes, hence only one pipe is open during normal driving). I agreed as I was not sure if Audi supplied a valve as a replacement part, let alone the cost.
I hate 'quads' & absolutely love the 'RS ovals', so it was a no-brainer to flange the valve/muffler/tips section so I could revert to OEM if required.