Early B-52s used turbojets, but the G and H models (which are all that are left in service) use low bypass turbofans. There have been a number of studies to re-engine the B-52 with more modern high-bypass turbofans (I was involved in a couple studies in the late 1990s using either the PW2000 or RB211-535 from the 757). The improvements in performance and fuel burn were astounding, but the program never go the go-ahead. One rumor was that the USAF brass were against a B-52 re-engine since it would make it harder to justify a large B-2 buy (which they ultimately didn't get either
).
BTW, the Bear may be able to top out at .87 Mach, it's cruise speed is less than .7 Mach, and it's burning plenty of fuel when it tops out at 0.87. The Bear's counter-rotating props mimic one of the other advantages I noted about turbofans - fan exit guide vanes - but at the penalty of huge noise.