Direct heated triode tube preamps and amps...

[cmalak]

Are any of the glass bottle mavens on AA familiar with "direct heated tube amps and preamps?" What is the main difference from a conventional design and what are the sonic differences? I know Wavac and Allnic (and I am sure many others) have some amps/preamps based on direct heated tubes but I am not sure what the differences are from conventional tube designs in terms of sonic signature, price, performance and realiability. They seem to be a lot more expensive that's for sure but not sure why.