mog00 has the right idea here. Losing the Subby is a must, frankly, but we'll actually break this down so that you can see how it works...
Rackbrute 6U = $359 street w/ 171 hp available (you lose 5 hp due to the P/S).
Subharmonicon = $699 street, takes 60 hp.
359/171 = $2.10 per hp.
$2.10 x 60 = $126, so...
The Subharmonicon, if it stays in the Rackbrute, now costs $825 because of having to figure in the 60 hp it takes up when it uses this cab.
Eurorack cases really are intended for modules that have no power and no cab of their own. Yes, I know that some synthesists like the convenience of that "all-in-1" idea, but this rig is also too small to allow for something like the Subby to take up nearly 30% of the cab. Plus, if you "take apart" the Subharmonicon down to its "primitives", the resulting list of modules would probably not take up that same 60 hp space.