It's difficult to read source code of XOOPS Cube because XOOPS Cube has many files and many classes. For reading source code of Cube, I recommend Doxygen. Doxygen is a famous document system, and generates HTML documents by parsing source code. Yes, that's a kind of javadoc. The generated document is really useful because Doxygen makes hyper links for surfing documents with a class name and a file name. You can do "Document-surfing" as well as Internet-surfing.
You may activate source code option, to output source code to your document. Generated source code pages include link also on a class name and constants. These pages are better than using IDE. You can look up classes and constants you wanted, easily.
Perhaps, you might stop using IDE to read source code even if that IDE is VisualStudio.
Because all classes don't have comments for document systems, the generated document doesn't have descriptions we wrote. But, I think the document is useful for you because Doxygen generates important informations automatically.