How Do Amps Affect Soundstage?

I've all tubes. I do find that they have a more pronounced three dimensional soundstage. It could be that they simply have a lower noise floor—allowing the tiny signals necessary for the most nuanced sound cues through. It could also be that the glass picks up sound waves which causes a slight reverberation giving the music a more echoic perception. 

Monoblocs sound better because there is no cross-modulation between channels. In other words, you get better stereo separation. 

I finally heard an amp paired with the 40.2 that actually made me want to sit and listen. The JFM. Sadly they don't give them away. 

It maktters , if you have a low budget amp say under $2k ,you will not get the budget soundstage and imaging that a good Solid state amp with bring 

which will have Mosfets, Fet, bipolors which as have some tube like traits .

IMO and many others a good SS amp , and Tube preamp is a classic combination 

a minimum of $10 k on average for a quality pre or amp ,

that would be -B class my audiophile standards. 

I agree with several of the above comments, and most seem to agree that the lower the distortion, the more accurately the source material, including the soundstage, can be reproduced.

Along those lines, I would say that vibration control, and especially of speakers, can also contribute to a better soundstage. Speaker vibration tends to produce distortion that is referred to as a "smearing" of the sound, and controlling it can reduce or eliminate the issue. Among other positives, I noticed a better defined soundstage after having employed Isoacoustic Gaia under my speakers.

There are numerous significant electrical and physical differences between a stereo amp and a pair of "identical circuitry" monoblocks. Here are four:

One electrical difference is crosstalk (L signal getting into R, and vice versa). Crosstalk is the result of inadvertent "leaking" of signal from one channel into the other. Two physically separate amplifiers are going to have less crosstalk if all other things are "equal".

Another difference is power supply loading. When electrical demands presented by output devices in one channel create fluctuations in available power, performance of the other channel will be degraded. Physically separate amplifiers have more electrical distance between their separate power supplies, and again, all other things being equal, the separate amps perform better.

High currents in the chassis of a two-channel amplifier will modulate the electrical grounding within the stereo amp. Disturbances to electrical ground are often discussed as problems in precise amplification. 

Even chassis vibration, propagating from one channel to another in a stereo amplifier, may cross-modulate signals and degrade performance. This is most often discussed with tubes -- microphonics.