What was the most impactful addition to your system?

Even in a setting absent resource constraints, the bug to “upgrade” is limited by space, power plugs, connectors, WAF, etc. Therefore, most of us must prioritize our additions or upgrades in our systems. I’m frequently seeking opinions on what chnage had the greatest impact to your system. For me as a gray-haired audiophile, it was the addition of a pair of subwoofers - even with a high end system. I am not dismissing the audible improvements from high end cables, pc’s, cartridges, sound treatments, speakers, TT, room placements, etc. But my auditory peyote moment came with the subs. Wish I’d known decades ago 😊