In the recent versions of Winamp, they have updated the in_wm.dll file to only work with plugins containing the keyword "waveOut" or "DirectSound". If the selected output plugin does not contain these keywords, you will get the error "Please Select DirectSound Output Plugin to Play Windows Media Content." if you try playing a WMA file, whether protected or not.
It also appears that they have designed the plugin to crash Winamp if the output plugin does contain those keywords, but is neither of Winamp's regular waveOut or DirectSound output plugins. For example, if you were to try and edit the out_disk.dll such that Winamp sees it with the title "waveOut output v2.0.3" for example, Winamp will crash as soon as you try to play a WMA file, although it will play and use the modified plugin with other file types.
I have tried using the Output Stacker, but can confirm that Winamp (at least recent versions) will give the same errors when attempting to play WMA files also.
Another option that may be worth trying is to get an old WMA plugin for Winamp, such as the one on Rarewares. This one allows the use of the Disk Writer plugin, however I'm not sure if it will play Napster's music since it is about five years old. On the following site, go to the section "Alternative WMA plugins for Winamp". The file contains two out_wm.dll plugins (one in a folder called 'Peter'), so try the one that is in the top level folder:
If the above plugin does not work, probably your best option is to re-record the music as it plays such as using TuneBite, GoldWave, or any other utility that records audio. Just be sure to mute all the recording channels apart from Wave and master. If there is no Wave volume selection in the recording channel selection, then select 'Stereo Mix' and mute everything in the playback channel selection apart from 'Wave' and 'Master. While this may not give as perfect conversion as a direct WMA to Wave conversion, any hiss or distortion introduced is generally much less than the audible artefacts I notice in the original WMA file anyway.