You can use any lossless codec for DTS encoded wavs! FLAC for example. Might be worth trying some!because Wavpack is AFAIK the only Lossless-Kompressor, which can handle more then 2 Channels. I think, the only way is back to Wav-Format - without Tagging !
.apl should work.One Question: Whats about support for .apl files?
Make sure .apl is listed in MP music config extensions (to have them listed in MP music screens) and in the extensionlist of the PureAudio plugin (should be there already).