You could always try moving your speakers to about 1 foot apart

This won't be an ideal solution, but, if you take the + lead from the left channel, and the + lead from the right channel, and hook those + leads up to a speaker in the center, you'll get a cancellation of anything common to both channels (which would appear to be the rhythm section if the info is exactly the same) and the unique info in the left and right channels will be summed, giving you an approximation of mono.

Back to Back, like its companion album Side By Side was recorded in stereo IIRC in the late '50s (1959 or so). I've heard it sound pretty freaking incredible, although not on CD, which was pretty poor as I recall. Acoustic Sounds has a pretty good LP, again if my meory serves me well,

B2B and SXS are great late Ellingtonia, with Hodges deeply digging into the blues. These records, like the great Hodges "Castle Rock" are fantastic '50s danceable blues/jazz.

Sorry to hear you've run into a trabsfer that dilutes that.