Look on ebay for writing desks and Welsh dressers.
They are pretty much unsellable especially if they are dark brown wood, made in the 70s or 80s, and definitely if they are huge.
Convert the draw fronts into drop down flaps to reveal a sliding tray for a electric piano or mixing desk, put a rack unit and amplifier into the cupboard on the bottom.
If the top unit has glass doors, hang curtains inside to obscure the PC monitor and powered monitors. Go big on lots of shelves and alcoves for storage - as you have already discovered, having one single 8'×4' shelf leads to a messy layout.
Try also local auctioDepending ouses, Facebook market place, and house clearance companies. Even if they don't want that sort of furniture, they might be willing to tell you about it.
If you buy yourself a Victorian piano stool to place in front of the unit, you will have somewhere to sit that matches the unit.
Depending on the layout and size of the unit you could use it to store bass guitars. Perhaps you might buy a second unit (a wardrobe) to store the instruments.