Good to know. That 553 sounds like a damned good option and I'll definitely look into it.
Since you mentioned low light...
I don't know how others use their spotters, but I find it most useful as a means to confirm that a suspicious object in my binos is a deer or to determine legality of bucks during daylight hours.
I don't use my spotter near first/last light, as I am spot and stalk bow hunting and often times, I find that:
A) If I can't resolve the deer in my binos around last light, then it's probably too far away to mount a reasonable stalk in time for the kill to be during legal hours.
B) During first light hours, the muleys are usually still up and about feeding in the center or at the edge of open areas or ag fields or they're beginning to move back to bedding in the sage, so a lot of times, they're visible with the naked eye and clearly visibly using binos.
Maybe I'm doing it all wrong, as I'm still new and learning and don't have a more experienced hunter as a mentor, but this has caused me to think that the low-light capability of spotters isn't particularly important to me.
I have considered going to a larger objective over the 55-65mm only because I had thought that the daytime light gathering will also improve image clarity at higher zoom levels. This is where I find the biggest disadvantage with my current Diamondback spotter. It's pretty much useless beyond about 30X even in broad daylight.