Q:

Should I prioritise reach over stack (in choosing a new bike)?

I'm in progress of choosing a new bike (or more specifically trading my MTB for a CX), and have asked for a CX with stack and reach numbers fitting my current road bike, an BMC GF01 'endurance' bike (I ride it with the stem low with only a conical spacer under it, and I'm very comfortable on it, even on full day rides). 
size 51: tt 527mm, reach 365mm, stack 546mm, st angle 73.5
However it turns out that the reach of 365mm for a '51' frame is pretty 'short' in today's geometry standards.
Is my thinking valid, prioritising reach to match, and sacrifice height on the stack? Or should I keep looking for a close-to-fit on both numbers?
Of course a lower stack means I can get lower and as a result more stretched (not that I want to), but too low means an awful lot of spacers which is a no-no.
For reference some example numbers.
BMC CX size 48: tt 518mm, reach 368mm, stack 506mm (reach ok, but stack is 40mm! lower)
BMC CX size 51: tt 534mm, reach 377mm, stack 530mm (reach is over 10mm! longer)
BMC RM size 51: tt 539mm, reach 376mm, stack 550mm (-"-)
Cannondale Super X size 48: tt 505mm, reach: 365mm, stack: 515mm (reach ok, but stack is low)
Giant XTC size S: tt 530mm, reach 370mm, stack 540mm (reach is 5mm longer)
Focus Mares size 51: tt 534mm, reach 365mm, stack 537mm (close to my GF01 geo)

A:

Endurance geo tends to be shorter reach with a higher stack, but variation on the recipe between brands.
Cyclocross bikes are race bikes (although years back they were often marketed as do all bikes, which gravel/all road have now taken over). Race bikes tend to have a lower longer cockpit for a given size, with variation between brands.
So it isn’t far comparing a Grand Fondo bike to a bunch of race bikes.
In terms of what to prioritize most bikes have a fit window, which is a range of final realized stack/reach coordinates after you adjust spacers, stem length and stem angle. Even handle bars have different reaches, which can further tune fit.
You have a lot of latitude with a simple stem swap. The 51 BMC CX Reach is 10 mm longer, which could mean running a 10 mm shorter stem. If you are planning to race you may find you prefer a slightly longer position.  
The size down 48 BMC CX could be harder, it has 40 mm less stack, moving all the spacers will only typically give you 30mm of stack on most mainstream bikes, which means you will have to flip the stem. Also note that spacers or flipping the stem up will reduce the reach (the head tube is on an angle).
Typically I will crunch the numbers in a spreadsheet to determine what stem, spacer combo is needed for a particular fit.

but too low means an awful lot of spacers which is a no-no.

Why is this a no-no? Are you riding a bike or worrying about style? People have different proportions. If for example you have a long inseam you lose stack height and will likely need spacers. A larger frame won’t handle as well due to weight  distribution differences.

