[Radioroslyn]

The single most effective way for large radios is the bucking transformer. If you want it inside the radio.
Basically it is simply a transformer that converts 120V to say 12V, and then the 120V AC is put through the secondary in counterphase so the 12V gets subtracted from 120V. Then it gets to the actual radio's transformer's primary.
The secondary has to be for the same current as the radio's primary.

http://dynacotubeaudio.forumotion.com/t3...ransformer

Read the thread. One of the folks pointed to Circuit Specialists who carry this: VARIAC-Auto Transformer 20 AMP
In the description it states, in part:

Rated at 110V input with an output voltage of 0-130V. Maximum output current is 20A or 2KVA. This auto transformer is VERY popular for home coffee roasting machines. It allows much greater control of the temperature of the roasting of coffee beans. They can also be used in science labs, audiophile (Great for old Tube Amplifiers), Science lab, hobby, film & video lighting applications. The auto transformer or Variac (both terms apply to the same item) is used to adjust the AC power either up or down. For instance if the AC power from the power company was 125 VAC and your application required 90 VAC, then this Auto transformer could be plugged in and the AC voltage could be adjusted to the desired level.

I'll admit that I'm greener than grass and definitely not an electrician. What does the 110v input rating refer to, since later it states that if the AC power is 125VAC you can adjust it down to 90, etc. Is this the right thing for what I'm looking for?

Using a variac is a bit overkill if your just want to drop the line voltage. As Mike mention a low voltage transformer wired to buck against the input work great and is pretty inexpensive. Found the transformer on epay.
http://www.ebay.com/itm/Basler-24vct-40v...SwpLNX9--D
It's good for about 2.5A @24vac. When wired properly if your line voltage is 125v the output would be 101vac. You could use a 120v to 12v transformer which would give you 113vac.