What is the advantage of ultra bandwidth?

To achieve wide bandwidth, the circuitry must be "fast". This means improved transient response, better dynamics, etc... It also means that the signal remains linear well beyond what we can hear, making in-band response much cleaner and purer. Limited bandwidth typically results in phase shifts within the audible frequency range along with high frequency roll off.

A good spec to look at amps with is called "power bandwidth". This is FAR more important than "frequency response". Frequency response is measured at 1 watt of output and power bandwidth is taken at the rated power output. If an amp is truly "fast" and "wide bandwidth", it should be able to do full power WAY, WAY beyond what we can hear. Too bad not that many manufacturers actually print this spec anymore : ) Sean
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In general, the wider the linear bandwidth of a system the better. Spectral makes very good equipment, but I would recommend that you start with their preamps and also be sure to use their recommended cables.

Sean, as usual, gives the concise & layman-friendly rendition of the matter. Onhw also gives good advice, I dare say.
I can add that music sounded VERY different to my wife's & my ears when we switched to wide-bandwidth amplification! The differences mainly concerned "background" (for wont of a better word) musical detail. We also felt that there was subjectively more time to listen to (more details of) the musical piece. Objectively, our previous equip is comparable, if not slower, than the wide-bandwidth -- or so the manufacturer claims.
Finally, as these designs amplify beyond 1MHz, ANYTHING in that spectrum will be duly served to your speakers for further... processing: this includes even harmonics (which we like) and trash (which we don't).
The DMA 100S: great product!
(I like Spectral, so my comments on their products are partisan!)