Sorry I was reading on my phone and misread your statement - I thought you said that the RK50 was cheaper than the khozmo, which is definitely not the case. Certainly the Khozmo will have better channel matching, but the rk50 should be good enough that you won't be able to hear a difference. The main advantages of the rk50 are that it has 'infinite' resolution unlike the stepped attenuator which can get into a case where one step is too loud and the other is too soft. It also has a much better 'feel,' I much prefer a smooth turning pot to the clicks of the stepped attenuator. I can't comment on the sound since I have never heard an rk50 but the khozmo sounds just fine to me. Last I checked rk50s were going for over $700 so its a pretty massive price gap that definitely makes the rk50 not worth it IMO.
For the best of both worlds check out amb's delta 1 stepped attenuator - 256 .5 dB steps so it is plenty fine grained to get desired volume, can be used with a RF remote control, and can also read a regular cheapo pot into an a/d to set the volume so you still get the pot 'feel.' Plus it can be built for about the same price as a khozmo if you use reasonable resistors. Takes up quite a bit of space and requires an additional 5V power supply which is why I didn't use one in my KGSSHV.