Just as a goof, not expecting good results, but would like to try a usb turntable we have lying around, into the usb input.
However, it's very difficult to get to the back of my 990.
Has anyone tryed this? I think at the very least, I'd have to get a usb gender changer...

I don't think it would work - the 990 is meant to connect to a computer as a peripheral, and so is the turntable

The 990 manual contains the following blurb on page 20, regarding the USB input:

Digital If your computer has a USB jack, you may connect it to the Model 990’s
USB input. You may have to use an adapter to convert the flatter USB-A jack
used on most computers to the squarish USB-B jack found on the Model 990.
Use the USB input only for stereo audio; if you attempt to play a Dolby Digital
or DTS soundtrack through it, it may produce loud noise that could damage
your speakers or your hearing. Also, the audio from the USB input will not
appear on the Model 990’s coaxial or digital audio outputs.

The manual definitely leads me to believe that the signal coming in has to be from a computer which has the necessary software to convert the USB turntable output into a signal usable by the 990. I have a Berhringer UFO202 which basically takes the analog output of my turntable and converts it to a digital signal. That signal goes to my computer into a file which can be read by software and digitized for conversion into .wav or MP3 formats.