Thanks! 
The TX6800 does seem to be the likely match for the SX580. I checked it out on the tuner info and stereomono sites. 
Receiver and tuner both came out in 79.  Looks like Pioneer put quite a bit of effort in making an affordable tuner with impressive performance. 
Seems to be a great bang for the buck tuner.  I might have to look for a bargain one to play with for myself.