To me it sounds like there is another sound channel. Ive listened to the CastleVania 3 soundtrack alot in my car, so when I hear the US nes cart version it sounds lacking.
I have an original power pack and it doesnt play famicom games that utilize that chip, but i think there is a mod for them.
Ive had my power pak for maybe like 8 years or so, and put it through some heavy use and it works perfectly every time. I would say that if you dont wanna buy the Pro version, buy a cheap/used flash cart instead; Id guess alot of people will be upgrading once newer flash cart models become available