Marble in the well is likely ...

Marble in the well is likely a short-term fix

Terrence, I'd be reluctant to put a chunk of marble in the well.  That may work for a little while, but I don't think it is a long-term solution to your problem. There's not much surface area with a single piece of stone, and surface area is needed to make the neutralization reaction go.  A common issue when using limestone to neutralize acidic water is that the surface of the limestone becomes coated with precipitates and stops working.  The geochemists call that passivation, and I think it could happen to your cube of marble.  The cube of marble may end up just sitting at the bottom of the well, while the water further up in the well is unaffected.  Then you'd put another chunk of marble in the well, then another... 

The above-ground treatment systems are probably a better choice in the long run.  You can replace the solid material and backwash when needed.  Don't use lime - it is a different chemical (calcium oxide) than limestone (calcium carbonate).  Too much lime can make your pH very high, which also can cause corrosion issues.  Limestone can't get natural water much above a pH of 8.3 or so.  

The calcium carbonate will likely make your water harder than it already is.  That can cause scaling and buildups in your pipes and your plumbing fixtures. It might be worthwhile to talk to a local water-treatment guy and see if adding a softener would make sense, or using something other than limestone to bring the pH up to 7 or so.

Good luck!