What is the best tweak?

First best tweak is washing all your vinyl with a good quality cleaning system. Many excellent systems exist. For the budget minded, the Disc Doctor system with manually operated vacuum machine is outstanding. You won't beleive your ears!
Second is running your entire system on its own dedicated electrical line from the breaker box. From a 20 amp breaker run 10/2 Romex up to two high quality 'hospital grade' outlets. Your components will appreciate their own food source and reward you with improved sound!
Regards,

DIY:tube traps,acoustic panels,RPG skyline diffusers.They were the icing on the cake.

Power cables. Never thought a ac cable would have as much impact as compared to isolation, speaker cables or anyother tweak. The ironic thing is the power cables were the least expensive of everything else. Go figure.