Monoblock tube amps are favoured for their ability to deliver extra power and provide less crosstalk (aka interference manifested as noise) when used as a pair. But monoblock amps dont normally support speakers and headphones as this one does.
According to Gizmag, Woo Audio's WA-234 monoblock headphone amp utilises a single-emitting triode design (preferred for minimal distortion), and has hand-wired circuitry to ensure quality. But what makes these amps especially neat is that they can support multiple types of vaccum tubes, enabling different sound profiles (typical tube amps don't do this).
And while this technical wizardry is impressive, they cost $US5000 apiece and of course you need to buy two (one for each channel). This pretty much means 99.9 per cent of the worlds population will never lay ears on these things. Such is the world of audio componentry.