What is your best kept secret in mid-fi?

Dedicated Circuits. Worth every penny in improved sonics and actually would have saved me time, money, and hassle if I would have had them installed sooner rather than later.

Invest in discs and LPs that are made using the very best recording and mixdown techniques and equipment. Although such media usually is more expensive than the average product, the resulting audio quality, even when played through a "MidFi" system, far exceeds the subtle improvements possible (?) when the typical disc/LP is played through multimegabuck equipment.

Give up the lowest frequencies (<40-50Hz), which allows you to use smaller speakers which in turn can be easier to drive. You'll get better performance above 40-50Hz for a reasonable cost, and your amplification will be cheaper as well.