Is there a digital audio input to the SA-AX720 or 730?I am trying to input a digital audio signal for 5.1 channel input and can't seem to find one. If there is not, what is the DSS in TV/DSS about?

Yes. You can hook it up using a digital coax or optical cable. Use can also use old stereo, analog, left and right channel audio. Use any audio input on your Technics, that will accept any of the inputs above.

In my experience, reasons for seeing errors in VST System Link (VSL) are usually due to the digital signal path (audio and/or data) not being routed properly within the System Link dialog box. If for example, you're using s/pdif between two ASIO 2 compatible audio cards properly (OUT of card #1, to IN of card #2 - then OUT of card #2 back IN of card #1), you also have to set your s/pdif System Link I/O channels in the dialog box to the 'same' channel...If say your options are s/pdif channels 9 & 10 (or whatever they are), you set the I/O's of both computers VSL dialog boxes to either channel 9, or all to channel 10. You of course need to route your digital audio properly within your audio interface! (see your cards documentation on that).
You can test if it's routed properly by 'listening' to audio from each computer, from the other...meaning listen at computer 2, from computer 2 & visa-versa. If you hear the audio thhen it should be correct. But in order to 'hear' the audio (besides proper settings in the VSL dialog box) you must 'also' set those digital audio s/pdif channels within Cubase's VST Inputs/connections. But in 'this case', you use both channels 9 & 10 for your Inputs/Outputs. They also say it's best to place your VSL digital audio channels at the top of the Inputs/Outputs list.
You do not need to keep the s/pdif channels in the VST Inputs/connections to make System Link work however, that's if you need to 'listen' or if you need to test whether it's routed properly.
Getting VST System Link working can seem complicated at 1st, but once you understand, and have it working, you realize it's actually not that hard at all. I recommend taking notes, and writing down what you did to make it work, as you go along the way. That has come in very handy when I need to use it again. Also READ your PDF manual and follow that to the letter!

In my experience, reasons for seeing errors in VST System Link (VSL) are usually due to the digital signal path (audio and/or data) not being routed properly within the System Link dialog box. If for example, you're using s/pdif between two ASIO 2 compatible audio cards properly (OUT of card #1, to IN of card #2 - then OUT of card #2 back IN of card #1), you also have to set your s/pdif System Link I/O channels in the dialog box to the 'same' channel...If say your options are s/pdif channels 9 & 10 (or whatever they are), you set the I/O's of both computers VSL dialog boxes to either channel 9, or all to channel 10. You of course need to route your digital audio properly within your audio interface! (see your cards documentation on that).
You can test if it's routed properly by 'listening' to audio from each computer, from the other...meaning listen at computer 2, from computer 2 & visa-versa. If you hear the audio thhen it should be correct. But in order to 'hear' the audio (besides proper settings in the VSL dialog box) you must 'also' set those digital audio s/pdif channels within Cubase's VST Inputs/connections. But in 'this case', you use both channels 9 & 10 for your Inputs/Outputs. They also say it's best to place your VSL digital audio channels at the top of the Inputs/Outputs list.
You do not need to keep the s/pdif channels in the VST Inputs/connections to make System Link work however, that's if you need to 'listen' or if you need to test whether it's routed properly.
Getting VST System Link working can seem complicated at 1st, but once you understand, and have it working, you realize it's actually not that hard at all. I recommend taking notes, and writing down what you did to make it work, as you go along the way. That has come in very handy when I need to use it again. Also READ your PDF manual and follow that to the letter!

Connect the 6-channel analog outputs of the AC500 to the 6-channel analog inputs on the AX720 using 3 RCA cable pairs.

http://archiwumallegro.pl/technics_shac500d_procesor_surround_ddigital_dts-1215399983.html

Digital sources will enter the AC500 and be converted to 5.1 analog output.

Be advised, that input on the receiver will not allow any internal DSP manipulation. WYSIWYG.

When you say you have sound from the BD prior to disc load... where (TV and/or receiver)? Initial wake-up audio is usually 2-channel.

Describe the optical audio connection input to the receiver and any other audio paths that are attached.

Do you have other multichannel sources you could use to test this, like a cable TV box?

There's an obscure setting in the receiver called PCM Fix (on Page 21). Give that a look.

Possibly a bad cable, but not likely its a 'wrong' one. Any RCA-style cable will do for SPDIF coaxial digital signals. Try another one. Are there other Digital Coaxial inputs you could try to prove the function?

You may have to configure your Digital Inputs so the receiver knows Digital Input 1 is Coaxial.

Manual usually has diagrams and pin configuration for your system.
If you don't have the manual go to their web site and take a look.

Audio? Video? input? output? Short questions are nice but vital details are helpful....
Lets assume your talking Audio output since those are the only digital settings you can change, but first a little bcakground.
RF would refer only to you video input via the tuner if thats what your referring to it should simply be a matter of changing the input (channel up/down or input up/down).
Your audio output can be changed from analog to digital and vise versa and can be chaged from the Menu(hit Menu when no DVD playing) settings under audio.