The question can't be answered because the answer really depends on the integrated at hand and the pre-power combo at hand, and then of course the peculairities of the physical setup and your application.
If gain is needed there are physical limitations to the integrated and the pre-pwr is the next tier up. However, some pre-pwr are not as good as some integrateds and vice versa. And when you have to sell a product, designers probably get caught up in what's going to sell versus what would've been the best sound for the money in a finished product.
If gain is needed there are physical limitations to the integrated and the pre-pwr is the next tier up. However, some pre-pwr are not as good as some integrateds and vice versa. And when you have to sell a product, designers probably get caught up in what's going to sell versus what would've been the best sound for the money in a finished product.