Ok; I put in a voltage divider on the 4v line so my soundcard gets approx 1v driving levels (using a 10k/39k drop), and this works at 0db fine. the 1v outputs are too much for a tape adapter, but I guess that they need less then a 1v drive, as it needs to be at around -12dB.

Still havent tried on a proper stereo input (to test the full 1v driving) but I suspec this will be Ok too. thanks guys for your advice