I'm currently using a Mark Levinson 390S direct to a pair of Sophia Electric 845 amps driven by 6SN7 tubes. I purchased a Haley a little over a year ago and connected it initially to the 390S only. It made an immediate improvement in detail retrieval, rendering of high frequencies, and overall quality of the music. Connecting the Sophias to the Haley in addition to the 390S improved the sound, but only marginally. The Sophias draw around 500 watts each at idle so the total combination didnt exceed the 1800 watt maximum for the Haley, but it was pushing it.
I received a Duke for my birthday (I have a very understanding wife!) and was absolutely astounded at the improvement it made when connecting the Sophias to it. The sound stage opened up tremendously; there was an incredible increase in "air"; and the bass was much faster, cleaner and tighter.
As you may know, the Duke now comes with a Cardas Golden Reference 20 amp power cord that has been manufactured to Running Springs Audio specifications. The Haley still comes with a stock power cord. I ordered a pair of 15 amp RSA modified Golden Reference cords with the intention of using them with my Sophias but they never made it to that position. After connecting one from the wall to the Haley, and the other from the Haley to the 390S, it became clear that they belonged in that position. ALL of the high frequency distortion I had been experiencing was gone, there was a noticeable increase in very tuneful, articulate bass; and the imaging improved significantly.
As it turns out, the difference between the RSA modified cord and the standard Golden Reference cord is substantial. I have a standard GR cord that Ive been rolling in and out, and in each position it rolls off the high frequencies (much like my experience with the GR interconnects). The standard cord may work well with certain solid state gear, but with tubes, I highly recommend the modified cord. It is more neutral than any cord Ive tried, and there have been many!
If you plan to use only one power conditioner, I would recommend that you utilize it on the front-end equipment rather than your amplifiers. While Im sure you will see a benefit by using it on your amps, it wont improve the quality of the signal they receive. If you are lucky enough to have RSA conditioners for your front end and your amps, you will be a very happy camper!
Good luck!