Are you referring to the ultra expensive NOS Telefunken 803S and not the reissues that are 803S in name only?
The true, old Telefunken 803S is, at least in my phonostage (Viva Fono) and extremely clear, detailed, dynamic, and extended range tube. It is NOT for someone looking for warm and lush. In that sense, it is like old Telefunken 12AX7, though I would say it is more dynamic and a touch more lean sounding. In my application, it is dead quiet, but, then too, the Telefunken 12AX7s that I had were also pretty quiet. The 803S has a reputation for being extremely robust and having a really long life, though I don't know if the McIntosh is hard or easy on tubes. If it is rough on tubes, I would say stay away because 803S are now VERY expensive.