The fact that the speakers are sealed will certainly make them better candidates for pseudo in-wall mounting. Ethan's point is very important regarding a small and consistent distance from the speakers to the enclosure. If the spaces become too large, they will audibly resonate based on their geometry, essentially setting up standing waves as a room does.
I would think that 703 would be as or more effective but without knowing the exact dimensions of the speaker and hence the dimensions of the spaces around them, it would be guessing. You would need to calculate what frequencies would be reinforced by the space around the cabinets (which could be complex to do accurately) and find out which material has a better absorption coefficient at those frequencies. 703 is more linear at most frequencies than R-19, so I'd go for the 703 to be safe. That doesn't mean it would be best, but it will be much better than doing nothing. If you have small gaps as Ethan suggests though, you'd have a hard time slicing the 703 that thin.
Ideally, you'd have built the cavity as a hole to the exact dimensions of the front of the speaker, secure the speaker flush with the front of that angled wall, and then treat the larger area behind it with insulation instead of making a cavity for it to sit in. Then you'd avoid this issue.
