Hi, i have the same problem. Apparently Audacity can export Amiga IFF Samples in 8bit but my results were the same as yours. Of course some quality loss is inavoidable. I also tried to convert the samples on the Amiga side, using various Aminet tools. But somehow the most effective way until now seemed to be resampling. If someone has some hints, i would be thankfull too!
By the way, Octamed SS can playback 16bit wave samples, if i remember correctly.