I have got my Darkvoice now. For me its a massive improvement - i'm really loving the tube sound.
i can't say if its better than the other option above, but i will say that if you get one you will be very pleased with it and won't regret it.
there are no 'problems' using this amp with the 595s (bass roll off due to an impedance missmatch ect)
the improvements are quite hard for me to identify since its better in pretty much every way
the biggest improvement is with the mids - the 595 was nice for vocals before, with the darkvoice its very very smooth and liquid sounding.
for me the darkvoice gives a bit more bass - bass goes deeper and its a bit fuller. personally i wouldnt want much more bass, if any but i can see how it might be a bit bass shy for some - it also depends what music you listen to: with bass heavy recordings there is more than enough bass for anyone imho - its with less bass heavy music that bass heads might have a problem. this is part of the 595s sound signature and amping can only change it so much.
trebel seems to have a sparkle and magic it didn't have before.
edit: i just want to add that the build quality is really excelent and that i'm only using the stock tubes at the moment - it's supposed to be even better with some higher quality tubes.
hope that helps a bit.
i'm confident you will enjoy the darkvioce if you get one.