I also took a long time to find the just-right used tandem. We were riding a 40+ pound Burley for 10+ years, and spent a couple years looking for a new-to-us used tandem that was just right. Ended up getting a barely used Calfee (from Twocicle, who posts here a good bit) for about half of list-price. We then price shopped every component, mostly new, but a few lightly used and did a lot of price-to-weight tradeoffs to keep it within our budget ($7k total). Ended up working out perfectly. Still a lot of money, and a lot more than we've ever spent on a bike, but its going on 9 years with barely anything replaced except worn tires, worn chains, 1 worn saddle and 1 bar (for fit/comfort).
We love this bike, and while we are a bit faster than we were on the Burley, it's not drastic, and it's mostly due to fitness, not the machine. The Burley was almost the same amount of fun (other than me disliking the lever hood shape). You don't have to spend a fortune to have a wonderful and fast time, but niche bikes with niche parts do get expensive.