Have you considered Lyrita DHT? It’s well-regarded and at reasonable price (if you can extend the budget to 40k, or get deals on preowned - they keep coming on the forum). It works well in tandem with my Audiolab. You can check with Virenji on compatibility with your vintage Quad. He could also customise where necessary and possible.
The DHT preamp is very good. However, do check the input impedance of your power amp for matching. The preamp has a volume control at the output, implying a high output impedance of around 5k ohms. This can be changed if you're buying a new one and plan to use with a low impedance power amp