No experience with the 66x but I do have a STX 65 and also a Kowa 554. I also only use my spotting scopes for confirmations and not for glassing.
These are my personal observations with in-field use whether it is scouting or hunting. This may or may not reflect others' experiences. Like all other optics, you are going to compromise somewhere (if anyone ever tells you that a specific optic is perfect for all things, they're an idiot).
Hauled both my STX 65 and Kowa 554 over an extended continuous period of time on a black bear hunt. This hunt covered a variety of terrain, lighting, and weather. The 554 held its own against the STX 65 under most conditions.
Took my 554 to Kodiak to chase mountain goats. Was able to compare my 554 with the guide's older STS 65. The 554 gave up nothing to that specific spotting scope. No issues field judging the bears out several miles; not enough magnification to do the same with blacktail or goat (same with the STS 65).
The 554 and its fluorite glass are nice. The fact that the 554 is literally 1/2 the weight of my STX 65 makes it a no brainer for a lot of my hunts. It is not the scope, neither is the STX 65, for when I need to count inches with a partially or mostly obscured animal. Hell there are times my STX 95 is "lacking".
I would not pay full retail price for the Kowa 55x as that gets you up to the next tier. But when you can readily get it for about $1400 during the year with a little bit of leg work, then it becomes significantly more enticing.
