Same watts at 8 and 4 ohms?

     Better amps have very robust power supplies that can provide the extra current required to double the power.  For example, an old 125/250/400 Audire Forte at 8/4/2 ohms respectively, has one 500 watt transformer and 4 26,000 mf filter caps per channel.  A very fine Bryston 200 wpc amp, of that era, has a 375 wpc transformer, and 2 4000 mf caps per channel.  Guess which one the bass drops out of at higher volumes, or with more demanding speakers.
     A pal bought a set of B&W DM 14's from me for his 125 wpc Marantz super receiver from the same era. He decided to get a second set, like he had heard years ago at my house, powered by this 125 wpc Audire amp.  The Marantz absolutely would not push all four to nearly the same volume, and it lost a great deal of bass, and pinpoint accuracy.  This reciever is stuffed with a tuner, a preamp, all kinds of switch gear, an FM dial, etc.  There was no room for a big power supply.  
     Ever wonder how a Marantz A/V receiver can push all those channels at 125 watts per?  I have one, It can't. It can move the sound around, but that is all.  A separate sub amp is absolutely required for this type of thing. 

     Two words:  Marginal power supply.  Compare a Superamp of its era, such as a 125 wpc Marantz, or a 125 wpc Audire or 200 wpc Brystom amp and you will hear. 
     Taking up space inside the Marantz is a pretty decent amp. for what it is, but it also has a tuner and dial, a preamp, all kinds of switch gear, etc. Inside the Bryston are two 375  watt transformers and 8 4,000 mf filter caps, plus 8 output transistors with heat sinks.  The Audire has 2 500 watt transformers, 8 26,000 mf filter caps, and 12 outputs. All newer Audire amps have many more outputs.  The sound is what you would expect, as is the actual power available.  Running 4 B&W's through these three, the first thing you notice is how much bass the Marantz loses at higher volumes, followed by the Bryston and the Audire.  The Audire is rated at 125/250/ 400 wpc at 8/4/2 ohms respectively, the Bryston about 300 at 4 ohms, and is not designed to run at 2 ohms. The Marantz just a wee bit more than 125 at 4 ohms, and shows it when pushed.
     Why do you think a 125watt x 5  A/V receiver (even Marantz) cannot pus a single set of speakers and a non-powered sub? 
     Weight can matter.  more stuff weights more, especially with big heat sinks.     Interestingly, my recently refurbished (At Bryston!) Bryston preamp runs hotter than my Audire amps did at full tilt pushing 4 inefficient. speakers. 

Danvignau what is your point none of those amps even come close to doubling down when impedance is halved. My krell has a 5kva transformer and 260,000 uf of capacitance and 56 output devices but it even can't technically double down on power with halved impedance.

Better amps have very robust power supplies that can provide the extra current required to double the power.

As I said before it’s not just the power supply, but the output stage as well. Especially the use of BJT (bi-polar) output stages in complimentary NPN/PNP push pull configuration over using complimentary Mosfets as the P channel is Mosfets weak one the N Channel is fine.

My krell has a 5kva transformer and 260,000 uf of capacitance and 56 output devices but it even can’t technically double down on power with halved impedance.

But it comes close

Cheers George

It does come close, but I need another one to vertically bi-amp my apogee duetta 2 speakers, they are very inefficient but sound great.