scandinavia express is still operating, but it's true it has gone into liquidation. i used them to get from dar es salaam to arusha, a good choice from arusha to nairobi is Riverside and maybe you could check out Dar Express?
They are out of liquidation and still the best generla bus service in Tanzania. Hood is also good and really now offers a comparable service. Good for getting down south from Arusha and Moshi to expore the more exciting Southern area of Tanzania