Rockyboy, many "tweakers" simply minimize excess parts ( simplifying the circuitry and shortening the circuit path ) and end up going to higher grade passive parts ( resistors, capacitors, inductors, wire, rectifiers, etc... ) for the parts that are left in the circuit.
This in itself can make a rather stunning difference in my opinion. Most of the gains are typically in the areas of tranparency, liquidity, ease of "flow" and a natural presentation, increased detail with a complete lack or harshness or grain, better bass impact and definition, lower noise floor, greater extension at both frequency extremes, etc... If this sounds like a whole new piece of gear, in many situations, it really is. Then again, you have to start out with what is a good and solid foundation to build such a product upon.
As such, i would suggest talking to some of those folks mentioned in this thread along with a few others to see if your player really can compete with current models. They should be able to give you a "yes" or "no" answer. If it is yes, you'll need to know how much time and money would be involved with doing something like that.
Once you've got a general consensus and know the price ranges involved if you were to do such a thing, you'll be better informed to make such a decision. It is best to know what you are getting into BEFORE jumping in with both feet. Sean