Why do some amplifiers throw a bigger soundstage than others?

Two things: lack of phase shift and aural effects having to do with the 2nd and 3rd harmonics, the latter of which can make an amp seem more detailed and spacious.

Thanks @atmasphere

The perception of a larger soundstage can be influenced by various factors, including the design and characteristics of the amplifier itself. Here are a few reasons why some amplifiers may be perceived to have a bigger soundstage:

1. Power and dynamics
2. Low distortion
3. Channel separation
4. Transient response
5. Room acoustics

Thanks @mapman

Isn’t it nice to get straightforward and technical answers rather than responses that it’s user-error or placebo effect? Lazy thinking which blames listeners first is so enervating.

+1 @hilde45 @atmasphere @mapman @mahgister

Thank you all for distilling things properly. These important factors are often overlooked and/or usurped by either hyperbole or oscilloscopes.

In the “everything matters” way of thinking, the only valid test setup is to use identical conditions for both DUTs, and to introduce nothing extra into the apparatus that isn’t required for critical listening…that means no ABX switches, disconnecting and reassembling hookups, etc., which takes time.  Critically matching playback level is known to be a necessity in assessing quality.  How many of us can recall the sound well enough to judge in light of these factors?  I am in agreement with Jasonbourne on this one…I also believe the preamp is more instrumental in establishing soundstage than the power amp is in my experience.

I have one more theory. Amps differ in their ability to provide current into low impedance and phase shifted loads. The type of amp (class A for example) doesn't tell you how well it provides current. If the speakers in question have an impedance dip in part of the frequency range and one amp provides more current into low impedance loads, the speakers will have a different frequency response between one amp and the other. This difference in frequency response could explain issues like a difference in soundstage.

I've heard a dramatic example of this. I have a pair of Thiel CS6 speakers that dip to around 2 ohms in part of the frequency spectrum. I run them with a Krell KSA 300S that doubles its power down to one ohm (300 watts @ 8 ohms, 2400 watts @ 1 ohm). When I took the amp in for recapping I tried hooking up my Onkyo A/V receiver to the Thiels. The results were laughable. The Onkyo sounded like an AM radio. The soundstage was flat as a pancake. I figured there would be a difference but I had no idea it would be that dramatic.

IMO, many if not most audiophiles do not fully appreciate the impact that speaker impedance curves have on the sound of a particular amplifier. Two amps with identical power at 8 ohms can sound radically different on a given set of speakers depending on the speakers' impedance curve and the ability of the amps to provide current into difficult loads.

I have a pair of Thiel CS6 speakers that dip to around 2 ohms in part of the frequency spectrum. I run them with a Krell KSA 300S that doubles its power down to one ohm (300 watts @ 8 ohms, 2400 watts @ 1 ohm). When I took the amp in for recapping I tried hooking up my Onkyo A/V receiver to the Thiels. The results were laughable. The Onkyo sounded like an AM radio. The soundstage was flat as a pancake. I figured there would be a difference but I had no idea it would be that dramatic.

@8th-note Your conclusion might not be accurate - its very likely that current had nothing to do with it.

To present a sound stage the mids and highs need to be reproduced properly. You usually don't need current for that since most speakers are fairly easy to drive in the mids and highs. Theils tend to have their weird impedance issues in the bass region, which has nothing to do with presenting a sound stage.

If the Onkyo has a power amp input I would run your preamp into that and see if you still say the same thing. Of course by now the issue might be moot since this is an anecdote; my point is that there were a lot more variables going on here than you initially surmised.

Our OTLs are known for a very good soundstage and yet they have a high output impedance so can't deliver the power into lower impedances. Our class D can- but the sound stage of our class D vs our OTLs is nearly identical.