Eminence Delta10b, with its tiny Xmax, is an inadequate deep-bass driver.
I own several, bought at a firesale because they suck for this use, and don't sell.
Above 100 Hz, they really shine, and offer that wonderful 16 ohm advantage.
Two parallel 10B combined with a compression driver and horn at 1600 Hz makes a very loud top box that uses standard 8 ohm crossovers. Nice.
If your Marshall situation runs the 2x10 above 100 Hz (very doubtful) that would be the best use.
However, these would far outrun the 15, and output from the 15 below 100 Hz would sound weak by comparison.
I say doubtful, because a filter or crossover at 100 Hz is quite large and the components are very pricey.
The 4 ohm load confirms no crossover, because all drivers are in parallel.
You could put a HP filter on the 2x10, but it will be costly.