power generation (watts) and speaker load (ohm) are not really related. A McIntosh will typically have the ability to push a 4, 8, or 16 ohm load... and 50 watts (really closer to 65-70 if healthy) will always be as loud as 50 watts.
If your two jbl's are 16 ohm, you would wire them in parallel and have an 8 ohm total load.
If you put a 16 ohm load into the 8 ohm connection, you will lose some volume as well as head room, but that's not what I read you asking.
Actually you are harder on the amp with a lighter load. A lot of people will carry a 4 ohm load, and even a 2 ohm load. That makes the amp run hotter, and shortens the life of the amp. I never run less than an 8 ohm load. That said, you do lose volume with a heavier load, but your amp is not pushing harder, just getting less response.
Gone are the days we stopped to decide where we should go, we just ride...