I owned the 31SE for 2 years after going through various ARC preamps the previous 15 years. And having heard the Calypso (with NOS tubes) in my system, I agree on your assessment: I much prefered the 31SE on the lower half due to its greater portrayal of space and decays and midrange fullness, but the Calypso brought so much more detail and openness on the top. It would be a tough choice here coming down to one's priority for one set of strengths over the other.
I seriously considered the 51SE but once I dropped in the Aesthetix Callisto Signature, there was no desire to continue the preamp quest. With the LS5 II over the SP-10, the LS5 III over the LS5 II and the 31SE over the LS5 III, each had been marked improvements over the predecessor. In the case of the Callisto over the 31SE, the Callisto was simply in a whole new class altogether....and this was with the stock Sovtek tubes. I had not heard such a difference since I first heard the SP-10 pretty much destroy all its competition back in the early 80s. To try and describe the refinements here would not do the Callisto justice. And then to replace all of the stock tubes with various Mullard, Telefunken and RCA's, the Callisto improves by nearly the same magnitude. This is simply not an option with the super-tube based BAT models. And once I got super quiet tubes in the Callisto, it was just as quiet as the 31SE. I had both for a couple months until I sold the 31SE.
Not having heard the 51SE, I have no direct experience to the relative improvements over the 31SE. The previous posts here have given us an idea as to the improvements with the 51SE. And it is good to learn that the 31SE's soft top end has been improved with the 51SE. But my gut feeling is that the benefit here is on a similar level of improvements I heard with my previous preamp upgrades.
Yes, the Callisto is a pain with its 2 or even 3 chassis, the multitude of tubes, its clunky volume controls, and the lack of the super cool user interface and multi-function remote of the BAT. But once you hear this new level, you quickly do not care about losing the features of the BAT. I would love to find a full function preamp to get rid of the multiple chassis Io and Callisto but until something more compact comes even close to the performance, I will put up with the hassles.
The 51SE seems to be selling in the $4500-5000 range on the used market. I have seen the Callisto Sig with one PS sell in the range of $5500-6000. For $1000 more, I think it would be worth your efforts to try and hear one before you lock into the 51SE.