Balanced inputs are a matter of some debate - some recommend them highly, others recommend against them unless you have unusual circumstances that justify them. As clo2016 points out, the 755 and 770 are being phased out and will be replaced later this fall with new amps that we expect to include balanced inputs. On the one hand, if you are definitely in favor of a balanced arrangement, you should have several options from Outlaw to choose between. On the other hand, both the 755 and 770 have been marked down recently and can currently be considered a great bargain.
It seems to me that the discussion of 990 vs Denon is unrelated to the balanced outputs (since the 3806 doesn't have them). The 3806's feature list is respectable - very comparable to the 990's - but I don't know how it sounds compared to the 990 (which I think very highly of). The 990/755 combo is both a very good value from a cost standpoint ($2150 for a processor that sells for $1100 and an amp that originally sold for $1300) and a very good performer irrespective of price.