What will raising the Va of atransformers provide?

Firstly lets get this Class-A and AB thing sorted out.

Class-A: This is where the transistors that operate both the negative and positive going parts of the waveform are ALWAYS on. The reason why Class-A amplifiers get hot is because the transistors are always ON.

Class AB: is where the transistors are only on half of the time, they switch on and off, cycling between the two halves of the waveform. The good thing here is that the transistors are OFF half the time hence less heat.
The problem now is crossover distortion! This is where the 2 halves 'don't quite meet up properly'.

Stanwal - it has nothing to do with full power or AB varying with demand.

So the question to whether a larger VA rating can improve the audio is YES. This is partly because as the VA rating increases the transformer 'regulation' becomes more efficient. Also (and remember VA = Volt Amperes) as you increase VA your current 'tap' increases allowing your amplifier to draw more current when needed.
If you imagine - a standard desktop computer draws approx 25-35A for a few milli secs when asked to perform a BIG number crunch. - this is the equivalent of a large transient in music (kick drum, bass guitar pluck, timpany on drugs)

How much current does a preamp need????

Well, I'm not a Naim Audio lover BUT if you listen to any of there preamps with their smaller power supplies and go to a bigger power supply the improvement is BIG. This is the same for any amplifier design (A,AB,B etc).
Class-D (again not a fan) improves with larger VA transformer - just ask Hypex

Remember, to get the best from a larger VA rated transformer you must upgrade the rectifiers and filter caps accordingly or else it's bottleneck time.

Stanwal

high-end audio reproduction doesn't work like that. If it did work the 'theoretical way' then a bigger power supply or larger VA rating on a transformer on a preamp would do absolutely nothing!

As we all pretty much know though, it does make a difference.

Your calculations do not take into account instantaneous/ transient current draw - the current draw during fast, large musical passages which happen in a few milliseconds. This is where a larger VA rating is needed. If you have ever built an amplifier whether Class-A or AB and calculated the exact power requirements and even been generous adding an extra 1.4 multiple, you will still hear an improvement doubling the VA rating.

If the theory and maths could explain everything then how does high-end audio work? Theory and maths does not explain differences in audio nor does it explain the reason why a larger supply helps the audio reproduction in just about every piece of audio equipment. Ask any audio designer...he'll/she'll be able to get so far but then after that it's all 'guess work'.

stanwal: I thought the origional question was just changing the transformer
Dirk: Changing the transformer to something larger is changing to a larger VA rating.

It is your original explanation to class-A and AB that is questionable. Class-AB 'varies with demand' and Class-A 'runs at full power' and also the idea that they are all large and expensive - they are not all large and expensive.

I'm not asking you to take anything I say onboard, just that it is not as simple as you make out - if it was then we would all have the same system.