Best sub $1200 headphone as complement to 99 Classics

Quad ERA-1

At this price point you should consider the focal clear.  I’ve owned $200 headphones I thought were great and like you splurged on a luxury headphone.  The Focal Clears are really a special experience.

For the budget mentioned, the Audeze LCD2 Fazors should be on your very short list especially if you have other phones in your system. If you were looking for just one set of phones with no others to trade off with, the Hifiman XV2s would be my choice, as they like and do all music well. Enjoy the music

Besides budget, do you have any other qualifers?  Sound signature, weight, comfort, cabling, etc.

I haven't heard the Focal Clear, so I'm not sure how much better they are, but the Focal Elex are very good and you'll have some money left over.  Fairly neutral, pretty comfortable, detailed, not too hard to drive and there are lots of aftermarket cabling options.

Besides budget, I tend to prefer something slightly warm and enveloping over supremely analytical... more interested in getting lost in the music and enjoying the sense of being there than finding that last shred of detail. Comfort is important to me, but only comfortable enough to not be distracted by the fit for an hour or two on a Sunday morning or after the kids are in bed as I realistically won’t have all-day listening sessions. Great soundstage and imaging in a friend’s 2-channel setup were what first drew me to hifi, so I consider those a must. Definitely not a bass head so superb bass response is much lower on the list than great midrange for vocals. I have a young one in the house so durability wouldn’t be a bad thing.

All that said, more than anything else I want something that will be interesting yet different than the 99 Classics rather than just a better version of the same sound. I don’t want to go down the rabbit hole of upgraded cables anytime soon and won’t have any options with balanced outputs until I get a dedicated amp. Currently have a dragonfly red (3.5mm) and the headphone output (1/4in) from my integrated amp to drive them.

Thanks for the additional information.  I'd suggest giving the Drop Sennheiser HD6XX a try.  $220.  I think they'll tick most of your boxes.  I have a number of much more expensive headphones (Audeze LCD-4, Senn HD800, Sony MDR-Z1R, Focal Elex among others, but reach for the HD6XX more often than not.