Replacement PSU. Original box and manual.
Here's what Blackstar have to say about their HT-Boost:
The HT-BOOST’s high voltage gain circuit is like adding two extrahot valve stages to your amp to ensure the maximum delivery of precious dynamics and tone. Other features include an all-valve cut and boost tone section and a high level output specially designed to drive your valve amp hard. This is an extremely flexible pedal and can be used into a clean amp as a volume boost, an additional valve equalisation, or to drive your amplifier into dynamic valve overdrive.