There is such a thing as "chronic low level depression". He means his depression is low-moderate and constant. In terns of DSM-IV-TR jargon his condition would be described as a Depressive Disorder that doesn't meet the criteria for Major Depressive Disorder. Dysthymic Disorder is one such Depressive Disorder.
By referring to a "chemical imbalance" he was trying to say (in lay terms) that his depressive illness is not due to exogenous factors and very likely due to a genetic presisposition. He is saying his chronic low mood isn't because he is unemployed, divorced, lonely etc.
If he is on venlafaxine it means his depressive illness has received a diagnosis and that he is under the management of at least his primary care physician. If the depressive illness is entirely endogenous in origin -- as the OP has indicated -- then nothing more can be done, counselling is not indicated. There is no "blood test" for depressive illness so I am unsure what your point is.
Further, I didn't advise the use of AAS I simply made a factual (as opposed to a normative) statement regarding the pharmacology of AAS and venlafaxine so your point about ethics is irrelevant.
This is just empty speculation. If anything being on gear will lift a low-level depression.
It isn't as if the OP is making a non-redemptive decision. For the sake of argument, lets say he has a adverse reaction to AAS. Then what? Simply stop taking the AAS. I don't understand your fatalistic conception of the matter.