I'm getting somewhere with music on Linux. One of my problems was that I apparently didn't have /dev/sequencer defined; a reinstall of ALSA (in the Debian standard make_kpkg modules_image manner) resolved that. I found some soundfonts at http://www.personalcopy.com/, some of which are decompressed for Linux use (most are self-extracting Windows executables). My SoundBlaster Audigy can load soundfonts with sfxload, so I don't need a softsynth.
So, I'm getting somewhere. But there are a lot of bugs in Rosegarden 4, or perhaps it's just the Debian-packaged version. So, I will be doing bug reports soon, if not looking at the code myself, before I can seriously use that tool. But that's ok; all software has bugs, it's just that I can actively participate in fixing these.
My frustration has at least temporarily abated, and I no longer feel any inclination to run out and buy a Mac.