Have I Hit The Point Of Diminishing Returns?

System ... Musical Fidelity Nu Vista CD, Bat VK-3i Preamp, Musical Fidelity A300cr power amp, Magnum Dynalab MD-102 Tuner, B&W N804 speakers, Cardas Golden Reference speaker (bi-wire) and ICs. I realize my rig is a bit dated, but it sounds great. If I were to upgrade, how much better could it get? Have I hit the point of diminishing returns where a lot more $$ gets only a small % increase in sound quality? If not, what component would you suggest upgrading and why? Thanks to all.