I've never had a quill stem bar slip; it is not simply their nature to creak. If you want to use an adapter and get modern 31.8 bars or something, nothing wrong with that, but you don't need to if your only issue is creak.

Perhaps good to try the loctite trick already mentioned to rule out your handlebar sleeve being loose. Probably the stem bolt just needs to be tighter. Grease the fixing bolt, and tighten it more. If that doesn't work, best fix is to get yourself a Cinelli 1A or Nitto stem. Make sure the bar and stem are designed for the same diameter 26.0 or whatever.

The old Cinelli 1/R stems would creak. The 1A stems do not. Never had an XA but I did have the 3TTT equivalent, and it was fine.