While I value power conditioners for surge protection and lightning protection including such lightning protection for FM and TV antennas (to which I add lightning arresters where the cable enters the house) I am skeptical of their ability to do anything the amplifier and preamplifier ripple filters do not do better. With a good ripple filter such as a pi network with an indictor the components filter full wave rectified 60 Hz (typically by two diodes, sometimes vacuum tube, and a center-tap transformer winding output). Double that frequency and they are twice as effective because capacative reactance is inversely proportional to frequency and inductive reactance is proportional to frequency. At frequencies higher than auditable, say, 20 kHz, this factor is 500/6 for a reduced fraction of 20,000/120, the fundamental frequency of a Fourier series of full wave rectified 60 Hz. I admit at radio frequencies capacitance between inductor windings begin to occur, but vacuum tubes used in audio are not likely to amplify these frequencies. The 833A which a very few people use for the final stage of their SET has a cutoff frequency of 20 mHz and other audio tubes are likely to be similar because they are not designed for VHF. And if RF passes through the inductor in the power supply ripple filter, it is certain to be shorted out in the output transformer windings of the amplifier.
Therefore I am not surprised if you hear no change from an expensive power conditioner. Still, I see no harm if they do not add to the price and if they add shorting effect to unusually high voltage spikes.