Exploring specific frequency distributions?

I have looked at several options for synthesizer software, and everything I have been able to find myself has an inconvenient amount of abstraction for what I want. I think I can manage with CSound, but it’s still not ideal.

What I would like to be able to do is just specify a list of specific frequencies and relative amplitudes (relative phase would also be nice, but not necessary), and play the resulting waveform.

Is there anything that will do that, or am I just going to have to buckle down and write it?