You should measure that with an AC multimeter while it's under load. I suspect it will be more than 6V. Whatever voltage it is, that's all you need if you're happy with the brightness. The same voltage as DC should give you the same brightness, as #12 has noted....from what I saw, 6VAC is bright!
It's also worth noting that 6VAC contains peaks to ~1.4X the nominal voltage, so if you rectify and add an electrolytic to smooth out ripple, with no DC load you'll see 6VAC * 1.4 - 1.4V(the diode voltage drop) = 7V DC. Confuses the heck out of noobs.