A website containing all "the usual suspects" of primary sources touching on Anglo-Saxon performance.
Written references from Tacitus through Widsith. Hucbald and Guido for theory. Manuscript illuminations. Archaeological finds of instruments. And a biblio of secondary sources and relevant oral poetry work in other cultures.
If for no other reason, the next time someone on the lyre list asks about the possibility that duets were played (or whatever), I'll have the relevant quotes handy instead of having to run home and find them in Opland.