The fisher 400 has that voltage doubling circuit and uses 7868 tubes; yet it sounds pretty good; especially with vintage equipment and shines with KLH speakers. The 500c seems a little less open and seamless, but has a beguiling sound in the midbass that is hard to dislike. Bizzybee does something to the 400 that changes the circuit and makes it one of the best sounding vintage pieces. Anyone like these units? It's nice to have a reciever of tube design in a second system. I will one day soon restore my 500 to try with Pi THeater 4's which should make a great combo since the Fisher rolls off slightly on the frequency extremes while the Pi 4's have a nice full range response that should compliment the 500c beautifully. I would love to hear of any experience with this set-up.
I have had the 400X (rebuilt by BizzyBee) for about 3 years now and it's the best piece of gear I've owned. The FM tuner is about as good as it gets. I used it with the JD-100 CD player and for speakers I have a set of JBL L-36s. The mods include replacing the 7868s with EL34s, and 12AT7s in driver/phase inverters - lower gain than the original 12AX7s, but plenty for the EL34s.