Hello, I'm new to the forum, but old to the audiophile game.
A good receiver for vintage speakers like yours would be
pretty much anything in the 500-800 dollar range from
Yamaha, Onkyo, or Harmon Kardon. Harmon Kardon especially
if most of your listening will be stereo. Great power, great
sound for stereo listening. Of course, they would also have surround
capabilities, video switching, etc. I mentioned the above brands,
because they would be the easiest to get at a decent
price, and in these price ranges, you will be getting "cleaner"
sounding power than with a cheapie $200 low end stereo receiver.
One thing you may want to remember is that newer
amplification technology is quite a bit more powerful than
what was generally available when the CS99's came out
(late seventies, I think) so be careful cranking up the volume.