I visited Cremona by train from Milan one time back in 2003. An easy day trip. Best to go early before the Italian rush hour when the station is packed. The museum on the stringed instrument museum is attached to the art museum and is reached by touring the art museum. No basses but fascinating info on stringed instrument making including a selection of Stradivarius's tools.
Note that the town closes for lunch other than the restaurants and is mostly closed at the weekend, with the exception of the museum. Back then the Lutier center had no basses on display but they called around and found one for me to view (which took soem doing on a Saturday).
I wanted to Visit Sr. Nolli's shop, but the day I was there he was away displaying at an exhibition somewhere, but I do remember passing his shop and noticing that it was conveniently located next to a bar that advertised live jazz.
Everything is reachable on foot from the station, but puchasing a map while in Milan is a good Idea, as it took a while to find one. Then it was a trip down narrow streets to reach the Lutier center and other points of intrest.
While in Venice there is a small church that has been changed to a musuem, which displays the Bottesini ( or Dragonetti) bass, together with a selection of historical DB bows. (I did this trip before taking up the double bass some years before the Milan trip). It also has a nice selection of classical European CDs including some solo double bass selections. If I can find the name of the church/Museum I will post (edit. it is the St Marcos Link in Anselm's post above), but it is one block back from the grand c****. Tourist information can help. They are very keen on musical tourism in Venice.
Sorry about the lack of detail, but I'm getting older and CRS syndrome is upon me!!
Have fun and pack comfortable shoes.
