Two true stories:
1995: Office with a number of desktop PCs and an HP LaserJet 4 laser printer sharing the same AC circuit. During the middle of the business day, when someone started printed, invariably one of the nearby PCs would lockup or spontaneously reboot. We purchased APC 1,250 watt line conditioners (basically a big, heavy transformer in a plastic box) for each of the PCs. No more PC problems when the printer started.
1999: Biotech laboratory. We had an instrument called a Nicomp from Particle Sizing Systems. This instrument had a reputation for burning people while they took sample vials in and out of the unit. (I'm highlighting the overall design quality of the device) After moving buildings, the Nicomp seemed to have problems outputting consistent data. (There were now a lot of Waters HPLCs on the same bench.) We purchased a Powerware On-Line Uninterruptible Power Supply (AC -> Batteries -> Inverter, 100% of the time) so that the Nicomp was effectively running off battery power. Powered by the on-line UPS, all the data consistency issues with the Nicomp disappeared.
In the first instance we were pretty confident the start-up current of the laser printer was dragging down the voltage of the AC circuit more than the PC power supplies could cope with. In the second instance, the Nicomp device likely had a poor quality power supply. In both instances, line conditioning solved the problems.
Would I put a line conditioner between the wall and my Parasound HCA-2200 II amplifier, with its two 1.2KVA transformers and 110,000 µf of power supply capacitors? Absolutely not! A line conditioner would impede the amplifier's power supply from doing it's job. (Pun intended) That doesn't stop me from having a whole house surge suppressor and plugging the amplifier into a
good quality surge suppressor/power bar.
Line conditioners can solve specific problems. With a "normal" AC mains supply, any properly designed equipment will not need a line conditioner. If you live in a region with regular power outages and/or use a backup generator, line conditioning and/or an uninterruptible power supply might be a good idea. If you don't appear to be having AC power quality problems, you should not need a line conditioner. If your concern is EMI/RF noise on the AC mains, a good quality power bar like a Tripp Lite Isobar can solve that type of issue for not a lot of money.
If you are looking for a good quality line conditioner, look at computer industry brands like APC, Eaton,
Tripp Lite, etc., as they offer superior value and performance compared to audiophile brands.
VERY IMPORTANT: Line Conditioners, Surge Suppressors and Uninterruptible Power Supplies (UPS) each do a different job. While many UPS and line conditioning devices advertise some surge suppression capability, they are NOT surge suppressors. Read this
Eaton article!