One of the worst culprits, and very easy to assess, is fluctuating line voltage. It's a serious problem if you use a tube amp with manual bias. Just stick the probes of a simple volt/ohm meter into your wall outlet at different times of the day and see what it reads. If it's always low (118-ish) or fluctuates between 118 and 123, then you're going to need a regenerator like an Exactpower. No conditioner will help, but a regenerator in addition to holding a steady 120, will do everything else a conditioner will do but without ruining dynamics, etc.
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