I don't think blaze is right about it not generating until you kill them. I've used pickpocket to check bards inventories before, and seen a Stradivarius in there.
I think how it works is that all bards inventories are static, and they will always have one musical instrument. If you kill them, and they don't drop their musical instrument, they will just respawn with the same one. But, if you kill them and they drop their musical instrument, when they generate next, they will have a new instrument, which might include a Stradivarius.
Obviously this makes kill them to get a strad a long and tedious process, but you can speed it up if you have pickpocket. By pickpocketing their musical instrument they will generate a new one, and you can repeat this until you get a strad.
If you don't have high pickpocket, you can wait for the winter festival in Noyel. The festival will generate 6+ fresh bards, killing them all gives you a very good chance of getting the strad. I got two strads from my first festival. If you don't get any, and you're not averse to some scummy gameplay, you can backup your save before entering Noyel and reload the backup if you don't get a strad.
So New Guy, does that mean the bow and the dagger are gone forever?
The dagger and the bow will both always spawn in Lomias' and Larnniere's inventory in the beginning of the game. You can get both artifacts at any time by killing them, unless you've told them to leave your house. You will encounter Lomias plenty of times in party time quests, so you shouldn't have any trouble getting his bow. But you will likely never encounter Larnniere again. Which is a real shame because her ether dagger totally rocks. It's probably the best weapon for fairy dual wielding with the lucky dagger.
That's why I always use the free nuke from the red blossom quest to kill those meddlesome elves in the beginning of the game.