I love the model 88 as much as the next guy, myybe more since I've owned one since '99, but no matter what you do, it'sgonna be limited by the internal amps (just a bit noisy) and the speakers. The whole package i well above average as is but I doubt tweaking will significantly improve it.
If you want a better CD player, use the "aux" inputs on the back to incorporate a transport and a DAC if you want.