I love my old HP tube oscillator... I think mine's a 200AB, because it covers the audio range only, but I don't have it handy at the moment to check.
PRR mentioned calibrated attenuator sets and "gain sets." Maybe I'm an anachronism, but I use 'em. I've got two Gates stereo gain sets, as well as some H-P calibrated 600-ohm attenuator sets. I scanned the manual for the Gates gain set, which can be found here.
The circuit could be DIY'd if one were sufficiently motivated. Probably easier just to hunt around for a used one, though. Gain sets are, or at least were, used by every broadcast station for their annual FCC-mandated "proofs" (proof-of-performance tests). Calibrated attenuator sets come up often on eBay. These sell for pretty cheap, usually.