It is okay to run at 4 ohms into an 8 ohm load and vice versa, as long as it is not more than double the load (4 into 16 is 4 times and probably not safe for long/at volume) then it is perfectly fine even for the long haul. Companies like Mesa and Hughes & Kettner will put it into writing for you. honestly you have a great 2x12 and will probably gain nothing by using the boss cab, but you can try it and find out. Also you can run the engl in stereo, use just one speaker, (8ohms from the head into 1 16 ohm speaker) and then you have a oversized 1x12 that gives you more tonal and recording options.