I've never built the Ruby Amp, but the sound quality is quite good on the jammer.
The Jammer can handle 8 to 32 ohm loads, not sure how a speaker will go, as I've never tried it in that way, though a speaker of no more than 1 watt should work well without further amplification.
As you've said, it is critical you use a stereo jack as the CD input, otherwise you will miss certain music components.
Even though the CD player signals are combined, it shouldn't affect the rhythm/lead components. It should include everything.
Please let us know how you get on.
Regards,
Relayer