Welcome back from lurking!
Did the channel imbalance happen out of the blue? Just so you know, I had a 404 Sig also develop channel imbalance with the left side being quieter to the right but I ended up selling it as repairs for it was just not worth it and it is impossible to buy spare transducers from anywhere else except the NSW Stax distributor which I got in contact with to get a quote, to me the fix was just not worth it at all. (BTW all other distributors from UK and USA don't stock 404 drivers). The same applies to the 303.
Here's the response from Nathan from AudioMarketing, Australia's Stax distributor.
So all in all, it is a better option to part the current 404 and buy another one or you can wait for someone to rock up with a pair of spare driver or beat up 404 so you can fix it yourself, but your luck is whim.
Do contact me if you're parting, I will be interested.