So to clarify - you still have the adapter cable
Something like this
And your speakers connected to your stereo that is gone, how did the speakers connect RCA, bare wire with banana plug? Can you change the connector or wire that runs to the speakers. You mention they are hard wired in place?
But yeah I would think a simple amplifier is what your after - budget? You mention that you want it small but don't mention any sort of budget..
Something like this maybe?
Product Dimensions: 4.7 x 5.8 x 1.7 inches ; 1 pounds
I am no audiophile by any means, but I don't think you can get sub woofer or 2.1 from a headphone jack? I don't think the low freq is broken out to its own channel, I believe it is just downsampled? if your speaker set? Has the connection to the sub this should work. If you need your own input to the sub then you would need a receiver that does the amplification and downsamples both channels (left, right) out to the sub.. Like I said I am no audiophile.. so maybe I am completely wrong there. But this is my understanding of how it would work with a sub and only left right channel inputs (2.0)
Maybe something like this if you need sub output