I bought a Zeller from Clarky for not a huge amount more than your £600 budget, absolutely lovely "cheap" bass which he was replacing with his dream DB, an 1880 German flatback costing him several times more than he sold the Zeller for.
He spent the next three years trying to persuade me to sell the Zeller back to him because it sounded nicer than his flatback. It's not always about price and reputation.
So is my Zeller a bit of a special, a one-off?
At the DB Bash a few months ago, someone else had one just like mine, so I tried it.
Even with his string choice and his set-up, it played and sounded a lot like mine.