I came across a new invention from Korg last night that got me super excited! In cooperation with Noritake Itron, Korg has developed the 21st century version of the good old vacuum tube. Having owned a modern tube-amp for a couple of years now, I can attest to the uncomparable sound quality.
Development of vacuum tubes pretty much stopped after the 1970, when the transistor was invented, due to several serious drawbacks: they're very fragile, they use a lot of electricity, most of which is converted into heat, which results in a relatively short life, ergo, you'll have to swap out the tubes several times during the life of your device. Korg appears to have addressed all of these issues.
According to the fact sheet they've put out, the Nutube uses less than 2% of the power of a traditional tube, which undoubtedly will keep it pretty cool. It's also less than 1/3 the size. This means it can be used in much smaller enclosures. At 30k hours of operation, it could even be soldered directly on to the board, since it'll likely last for the lifetime of your device.
Combine the Nutube with the very decent SMT sized DACs you can get these days and you can have a very good amp in a very small box. Portable and battery-powered aren't out of the realm of possibilities. (Read: headphone amp)
If this performs as advertised, can be produced in volume and at a reasonable price, this is the invention of the decade already, in my book.