Quad gear is excellent as others have said.
I’ve owned lots through the years, including the ESL63s and 989s, together with the amps, cd and tuners that Quad had in their range from the same time period - 33/303, 34 and 44, 405/2, and 909.
I still have a 99 series set up as a small secondary system.
Above all, Quad as a company will look after you, and are able to still service and repair the majority of their legacy equipment.
The 306 is a virtually bullet proof design, is flexible in what it can be partnered to, and sounds great.
Budget for between £200 - 300 per box, if working - but look out for cosmetic damage especially on the front fins of the 306, many of which have had tell tale knocks over the years!
Buy with complete confidence, sound in the knowledge that you will always be able to sell it on if the time comes (usually at no real loss) - AND - above all, never be left with an obsolete, unrepairable ‘door stop’.