My understanding is that the 4344 MKII crossover will get you the smoothest sound out of the 2123 for this type of application. The issue would be the crossover point of the 2245 would have to be raised to 340Hz from 290Hz as in the stock 4345 with a 2122.
Most people in the forum don’t like having the 2245 play this high.
There is a lot of information on the differences between the JBL vintage 10” speakers here,
http://www.audioheritage.org/vbullet...2121-2122-2123
as they are not considered to be that interchangeable.
There much discussion on the 4344 MKII crossover you’ll have to search the forum on your own as it is spread across multiple threads.
Chris