When I test a synth before buying it, and there is no amp available, I check the signal output(s) with my cheap walkman headphones, and I can pick up the sound. It's not enough nor fun to play long like this, but it's still a possibility...
Just tell me there is no possible damage to the synth itself (why could there be anyway?), and the rest is irrelevant to me.
-Roland JX3P+PG200, D50+PG1000, Juno 106, MKS50/70, JP8080, U220, MVS1, MDC1, MSE1, TR-707
-Korg M1R, MicroKorg, Micro X, Kaossilator, Monotron X3
-Yamaha DX7, TX816, TX81Z
-Future Retro 777, DSI Mopho, EMU Proteus FX