There is a lot of discussion around about getting 'good tone at low volume'
Not too much experience with Power Scaling, but other master volumes are something I've played with...
the best sounding MV is all going to be personal preference.
I think i can elaborate on what Twinfan said...
Bigger amps... say a Marshall. The average marshall master volume is feeding into the Phase Inverter. So if you're playing at low volumes, you probably aren't driving the PI or anything after it that hard and doing that has it's own tone (good or bad is up to you).
Now... If you have a Dual Pot 4 capacitor Post Phase Inverter Master Volume (such as in the older JCM800 2205), you can hear this PI/Power Valve relationship! if you turn the PPIMV right down (between the PI and Power Valves) then you loose volume and negative feedback, but if you also have a volume pot in front of the PI (this isn't how the old 2205 is set up) then you can crank it, and you'll hear the Phase Inverter get driven hard at lower volumes with less negative feedback coming into the amp, which (assuming I'm talking about a standard long tail pair) to me makes a kind of fizzy nasty sound in this situation.
If you have a big amp with negative feedback, usually associated with presence and depth controls, then as you decrease the volume using a PPIMV, you decrease the NFB back into the amp and that makes these controls less responsive and alters the tone of you amp. So PPIMV's can cause you to compromise on certain things.
so for me, that is why using a simple MV in a bigger amp is a bit better. I like having the use of Negative Feedback based controls which can compensate hi and low end in a big amp played (kind of) quietly.
smaller more simple amps might only have 1 or 2 gain stages and an output stage. They may only have a volume in the front of the preamp. Turning this up will drive the whole amp. in an amp like that you cant attenuate the signal before the PI to drive the earlier gain stage harder before driving the power stage. You're probably relying more on power valve breakup in those amps too. So if you want drive and lower volumes, PPIMV's are probably not the way to go as they will still decrease signal going into the power valve. Power Scaling or an Attenuator might be better.
Some amps do have 'better' master volumes than others, but that really means they have different volume controls and people have a personal preference each.
for example.
Older JCM800 2205 has a PPIMV which is a dual pot and four coupling capacitors sending signal to ground in the power stage.
Later JCM800 2205 has a master volume between the tone stack and PI. this is a very common setup.
Matchless Lightning has a cross-line PPIMV mixing out-of-phase signals together in the power stage to reduce volume.
Carlsboro 60 TC has a 'Limiter' which is a scale control ON the Phase Inverter which I think decreases volume but maintains perceived drive levels. I've never heard this, I think it's rarely used.
There is also the LarMar PPIMV which is apparently very good in certain amps. I've never heard this either.
They all work a bit differently. You can read up on all the ideas and then consider power scaling also.
In my opinion, I think trying to get the sound of a cranked 100watt amp through a 4x12 at bedroom level's isn't going to work. You might get a good sound, but it will be a different sound. I think one problem most people tend to forget while talking about PPIMV's, attenuators, and power scaling, is the speaker movement. Better to get an amp designed for great tone at low volume than get a big amp and try to get the SAME cranked tone at low sounds levels. If you want the second of those two, I think you're going to be disappointed unless you can compromise.
