I have the Furman IT REF 20i and the Furman SPR20i. The Furman Ref 20i conditions the power with a massive torriod isolation transformer and performs 4 to 1 power factor matching. Thus, there is an interaction between your equipment's power supplies and the what is delivered at the wall.
Hands down I can tell you in my highly resolving audio system that the difference was immediate as images lifted off the speakers. I am using my 15 amp house wiring circuit which is arguably terrible. I then nearly did not, but finally did pick up the SPR20i and connected this to the wall first, then used the high power outputs to the Furman Ref20i (which essentially has a capacitor inside of it "the size of your forearm"--to quote one Furman engineer or obvious to anyone with some spacial imagination regarding the unit internal layout to store energy.
So, what the system sees through the individual filtered isolated banks on the back of the Ref 20i is now highly filtered and a virtual instantaneous 88+ amps according to their specs and longer duration (I assume one cycle) 8+ amps.
When I added the SPR20i (speced at 55 Amps peak, which is just fine), the noise floor during music playback dropped noticably further due to the further filtering and the measured Voltage at the output became stable without short term peak noise.
I had measured peaks on the Fluke 87-V of around 500 to 700 Volts out of my wall socket. Then I did this at the Tripplite Power Distribution Unit (PDU) at my amp input and no peaks were measured. RMS voltage became filtered, smooth, and abundant at this output (into the equipment).
I went further and measured the monitoring taps on one of my Hi-End Tube amps...that are auto-biased, but allow one to monitor the health of the 300B PSETs Tubes via two test points per monoblock for a Volt meter, which is the analog of the milliAmps of the Tube filiment regulated auto-biased power supply circuit within these monoblocks.
Now, instead of the Voltage moving around a few Volts, especially over a time span of say 30 seconds, I got stable Voltage for periods of about 30 seconds, meaning that the Voltage did move, but without any spikes and very slowly.
This cleaned up Voltage for tubes is scientifically known to keep the filiments out of a state of stress by keeping the filiment Voltage very stable. Your expensive Tubes WILL last longer. You may hit a break even point right there, if you think about it that way.
It is better than mere measurements however, the sound is unbelievable and the improvement was not subtle.
I also have a MC452 SS amp and this opened the soundstage and became noticably wide open with instrument further differentiated. The soundstage going backwards to a refined ear would hear a more compact sound...some would say smaller...even congested.
This is not a mark against the MC452, which is an exemplary SS amplifier, rather it demonstrates just how badly I needed power conditioning in my situation and that there was a very good reason my system did not sound right to me...after large investments in just audio equipment itself.
The Furman Reference power conditioning equipment is something that I whole heartedly endorse and found after my own exhaustive search.
It delivers when you PLAY the music. Yes, you will hear a reduction in line hiss if you stick your ear up to the speaker without program material playing, but the difference IS when the music IS playing. I have no doubt that videophiles will find excellent results.
If you want real and serious power conditioning equipment, without questionable secrecy about what is in the expensive box and the accompanying VooDoo, this IS THE ONLY power conditioning equipment I personally recommend.
There may be other solutions...but, this is the one I choose.
Electronics Engineer (BSEE w/honors)
Practicing for 25 years
PS--I would expect the Ref 15i to be an excellent unit, just know that its current reserves are 3 to 4 amps (presumed one cycle minimum duration) and 55+ amps RMS instanteous reserve. That reads like a good piece of equipment to me, especially in light of my own experience with the Ref 20i).
If you can afford it...highly recommended to get both the Ref 15i or Ref20i and the SPR20i and connect the two units you choose the way I have explained.
If you have made a substantial investment in your system that has equipment worth several thousand dollars each, then I think this is cost justified and in the same league and I sincerly believe you will notice a difference.
PSS--This is only a hobby for me. I do not work for this manufacturer. Also, I am NOT advocating purchasing anything Furman, just the pieces I cite here. You must do your research and evaluate your application. Some of their lower level equipment has mixed reviews. Some love it...others do not... Good Luck!!!
PSS--If I had it to do over again, would I do anything different with respect to power? I would actually call this the next step I have procrastinated on doing, because it requires an installation I do not look forward to in the current home.
The answer would be...I would keep all of this excellent power conditioning equipment and next add some 20 amp circuits. Such circuits can make a profound difference. I have witnessed such a difference on equipment I have owned on circuits that were probably 20 amp minimum...maybe 30 amp or more...but, supposed to be 20 amp based on their socket installation.
This difference is not subtle, especially with amplification...especially Solid State (SS) amplification...particularly to bring Class A operation into its operating point with the speed of only a few minutes and not hours on a 15 amp circuit. In that instance, The sound literally dropped off the speakers and the soundstage exploded...left in a room with the speakers just standing there like strange obelisks dissociated from the music enveloping the listener...