These are not my words however it is very interesting reading. There has been much written about isolation transformers on Audiogon, seek your information.
There are a lot of different roads that lead to Rome and some may be better than others. Like anything else, it is a matter of personal preference, budget, etc...
As far as isolation transformers go, you want to look for the largest core that you can get. This will keep hysterisis distortion to a minimum due to reducing the potential for core saturation. This is true of either E-I type ( old school ) transformers or toroidals. Big core size translates to high weight, but that can be misleading. Some transformers use "end caps" or "shields" ( this is a good thing ) that also contribute to the total weight. As such, don't think that a higher weight figure makes a transformer "better" as one might have heavier end caps with a lighter core than a unit with very light end caps and a huge core. The only way to really see what is going on is to either remove the end caps and weigh the transformers and / or simply eyeball the size of the core itself.
Two other key factors are based on spec's. One is the amount of rejection or "level of isolation" that the transformer achieves from the input windings to the output windings. A good transformer is down below -120 dB's. It is not uncommon to see figures ranging from -120 to -150 on high quality isolation transformers. If you do some checking though, most anything with a spec like that will be an "old school" iron core transformer. There's a reason for this and i'll explain why and what the other pertinent spec is at the same time.
The reason that the iron core works better than a toroidal is that the iron core typically has a MUCH lower level of internal capacitance. A higher level of capacitance basically equates to more signal leakage / increased coupling between the primary and secondary windings of the transformer. Obviously, the more coupling between windings, the less isolation. As such, a "good" toriodal will typically display a rating of -80 to -85 dB's compared to the -120 dB or better rating of the iron core. In this case, the lower the number, the better.
As you can see, the "old school" iron core transformer is measurably better. What this translates into right off the bat is a much lower noise floor and "blacker background. Treble performance is also "typically" much cleaner with less glare, sibilance and smearing. As a side note, this is also true of transformers used within audio components themselves. Any "honest" audio manufacturer / designer will tell you the same thing. The reason that most companies use toroidals is that they cost a LOT less to buy / build, they weigh a LOT less and the manufacturer can get away with a lighter duty chassis, they have a LOT less potential for breaking loose and destroying the product from the inside out when shipping, the products are cheaper to ship due to the weight reduction in both the transformer and chassis, etc... The one "real" advantage that toroidals may have in terms of actual performance is that, depending on how they are wound, they offer the potential for less EMI or "magnetic spray" inside of the component. Since many manufacturers stick large toroidals very near signal circuitry, much of that benefit is drastically reduced.
Other than that, the other "specs" that you want to look at are power rating and the input and output voltages. Most isolation transformers that you would be interested in would be rated in KVA ( KiloVolt /Amps or "watts" ). To make things simple for you, a 1.8 KVA or 1800 watt isolation transformer would be rated for 15 amps at 120 volts. To figure out how many amps the transformer is good for, you take the KVA rating ( 1800 ) and divide it by the voltage that you'll be using it at ( 120 ). As such, 1800 divided by 120 = 15. If you were looking for a 20 amp transformer at 120 volts, you would need one rated for 2.4 - 2.5 KVA ( 2400 - 2500 watts ).
Now here's where it gets tricky. Much like audio gear, different manufacturers play games with spec's. It is quite possible for two isolation transformers rated for 2.5 KVA with similar levels of isolation to be capable of very different levels of low distortion operation. You see, one transformer may begin to saturate the core at 15 amps whereas the other one is good for a "real" 20 amps. That's why i said that you should look for "BIG" core size. If you aren't sure of whether or not the core of the transformer is big enough, figure that it is good for 60% of its' advertised rating and go from there. This will allow plenty of "headroom" in most circumstances and you should never have a problem. Most of the people that complain about "side effects" when using iso's were trying to pull too much power from them and that's why they ran into problems with them. Kind of like saying "i only need 10 watts to drive these speakers" and then buying an amp that is only good for 10 - 12 watts. Sure, it may play loud enough, but the amp is stressed due to running near capacity and so is the sound. Such is the same thing with a transformer i.e. so long as quality does not suffer, a greater quantity is always preferred.
As a side note, the cores on my 3.0 KVA transformers are larger than the ones i used to have, which were rated at 5.0 KVA each. While the 3 KVA's will not pass as much power individually, the power that they will pass has less potential for hysterisis distortion since it would be next to impossible to saturate the core, even at full load. Only problem with this scenario is that i had to go to more transformers to achieve the same total power rating.
As to whether or not you want to go this route is up to you. I have a lot of respect for what PS Audio has done with the PowerPlants, but this approach also has a LOT of drawbacks. It is VERY costly, very in-efficient, throws off a LOT of heat, etc...
Balanced power has some benefits to it too. I think that there was a recent review of a product of this nature in Stereophile.
Running 220 - 240 volts and then stepping it down to 110 - 120 at the equipment level is also worthwhile. If you can reconfigure your gear for direct 220 - 240 volt operation, that is even better.
Like i said, there are a LOT of different paths that one can take. Running dedicated lines does reduce the potential for noise within the house to find its' way into your system to some extent. It is an excellent starting point. Since the dedicated lines typically share the same AC main though, noise can still "communicate" between your house lines and the dedicated lines. On top of this, there is nothing between what is coming in through the AC lines and your dedicated lines. While you do have a lower resistance path through the dedicated lines, which results in more stable voltage and current delivery, you've also got the potential for "dedicated noise delivery" too. As such, you really have to do something to isolate noise from the outside lines feeding AC into the house and the lines feeding the rest of the house from "contaminating" your dedicated lines via crosstalk at the breaker box. How you choose to do that and what suits your budget is up to you. Most anything will be a step forward so long as the device that you use isn't easily saturated. If the device does saturate, it is introducing distortion into the line rather than cleaning it up. That's why i stressed maintaining a certain amount of "headroom" above and beyond what you would ever pull from the devices used for filtering and / or isolation.