Here's an overview of power amp and reactive load characteristics Steve put together to hopefully answer some of your questions. Enjoy!
Two things we see frequently batted around in these discussions are: "Flatness" and "transparency". Both terms are often used incorrectly and without a clear understanding of the subject.
So, what does "flat" actually mean? In the real world of guitar amp behavior, not a lot really. For the record, we do not claim that the Power Station is "flat". We do claim that it is "more neutral" than a typical guitar power amp stage. Why do we say that?
First, contrary to a typical guitar power amp stage, the relationship between the power tubes and output transformer in the Power Station is such that the power tubes operate in a very linear region of their ideal operating range. This tends to subdue characteristics that we normally associate with a particular tube, and that in turn, prevents the power amp stage from imposing its sonic personality onto the sound of the amp being attenuated (or boosted).
Secondly, tube power amp source (output) impedance is high relative to a SS power amp whether Class AB or D. Power amps with very low source impedance (usually along with a correspondingly high damping factor), can sound relatively stiff and lifeless as a guitar power amp because they exert a lot of control over the behavior of the speaker. Also, a SS power amp normally requires a much higher output capability in order to prevent the player from crossing over into distortion territory, which in any SS power amp is pretty objectionable. SS power amps that exhibit lower apparent clipping do so either because there is a lot of negative feedback or compression imposed on the design, or the power supply is heavily filtered in order to stay as clean as possible, either of which sounds, and especially feels, artificial. In contrast, a tube power amp is allowed to slip into distortion territory because it sounds musically pleasing when doing so. No surprise there.
A tube power amp, having a significantly higher source impedance allows the speaker to react more - and often more slowly. Greater speaker reactivity means that a small amount of voltage from the speaker reaction reflects back into the power amp, enhancing the feel and responsiveness of the amp. Tube amps also utilize negative feedback to tame some of this reactive interplay between the speaker and amp, but it's generally less effective at controlling the speaker because of the higher amplifier source impedance. In fact, the Presence and Depth controls on the Power Station vary this feedback at very high frequencies (Presence) and very low frequencies (Depth). This is how the PS is tuned to interact optimally with whatever speaker you may be using it with.
What the player experiences when reamping their amp with the PS is not a coloration of the sound necessarily, but the effect of hearing that sound at a higher or lower volume than you normally would without the PS. For example, when you play the amp by itself, your pick attack is based on the amps volume and how hard you are able to dig in. With the PS connected, you are now allowed all kinds of license with regard to pick attack and playing intensity, which naturally changes how you perceive tone, volume, distortion and frequency response. The thing that changed really is your playing relationship with the amp. Only problem is, once you hit full output, the power amp then imposes it's own compression and distortion character on the sound of your amp. A higher powered tube amp eliminates that problem, of course at a somewhat higher cost. We originally chose 50W because that seemed to be a good overall compromise between cost, weight and size, and we, I think correctly, assumed that most people would be using the PS to reign in an unruly amp. For those who want both control, and the dynamic snap of a higher powered amp with just a bit of the horns shaved off, we've developed the PS-100.
Now, about "transparency", there is no such thing as a "transparent" reactive load. Its very job is to excite non-linear behavior in your amp, the same way a speaker does. And with regard to frequency response, power output and distortion, the notion that one power amp is more transparent than another is entirely subjective. For example, technically, a SS power amp may be more sonically neutral or flat than a tube power amp, therefore more "transparent" on paper. But do we really want that? When your standalone amp is interacting with its own speaker, there is a lot of give and take between the two that creates the amps personality. Insert a flat, low impedance SS power amp into the stew and a lot of that feel and character goes away.
So...if the reactive load in the PS is supposed to fool the amp into thinking it's hooked up to a speaker, why should that feel and personality depend on a supposedly "flat" tube power amp? The primary reason for using a tube power amp in the PS is to restore the interactive behavior between the amp and speaker that is lost when the amp itself is no longer driving the speaker you are listening to. And when you are in a room, how the speaker is driven, how the speaker creates reflections in the room you are in, the volume, and how your ears perceive frequency response at different playing volumes (Fletcher-Munson Effect), all weigh on the final result.
Once you attenuate your amp, you're turning it into a different, more useful and more fun to play instrument. Some things you may like about the behavior about the amp may change, while most of what you gain is beneficial. You don't lose the things you like. You just put them on the back burner and you can restore them with the flick of the Bypass switch. Now with a 100W amp and two sets of controls, you can get closer to both ideals and keep the FX loop active to boot.
The PS cuts the interaction between your amp and the speaker cabinet. As far as your amp is concerned, the PS *is* the speaker cabinet. This is important because there is an interaction between the amp and its speaker load. This interaction is 2-fold:
The speaker load actually determines how the amp, itself, sounds. So each cabinet makes the amp sound different.
The speaker moves and reacts to the amp's power, and the amp reacts to the speaker's behavior. So there's a dynamic interaction that changes with volume.