I have had a few:
1. understanding how important a tonearm is in the vinyl reproduction system. I replaced a Rega tonearm with a Basis Vector. The difference was astounding, and sent me through a lengthy system upgrade to see what else could be improved.
2. doing some room treatments took my system to another level. The best bang for the buck upgrade
3. replacing my very good tonearm cable with a great tonearm cable. It made a huge difference. Far more than I could have imagined

