It's not my experience that Zu break-in is anything other than progressive and uni-directional. That is, it's a one-way street with no backtracking wih the singular exception of significant thermal changes.
Zu itself has had the experience of doing 200-400 hours of intense factory break-in only to ship speakers from Utah in the winter months and have them arrive sounding stiff and not broken in. Sean has contended in the past that break-in of the internal cabling (especially the dielectrics) is actually more of a factor than limbering of the driver, but cold exposure can make the driver break-in paramount. I'm in Southern California, so I probably don't have the same range of temperature swings as you, even if I left my windows and doors wide open for weeks. If nothing else changed, I have to guess this is your issue, and it will be temporary. Also, if you are using vinyl, the same thermal factors can truncate bass response of a phono cartridge, too. Assuming nothing has happened to your amplification, it seems a temporary problem associated with an environmental change that a hundred hours of any band using a large Marshall stack can overcome.
Phil