Hi Dave

With the HD500 set up as default Amp A will go to channel A on the DT-50 and Amp B will go to channel B when connected via L6 Link.

Dual channel setups take Channel A on the first DT-50 in line.

Provided both amp slots have an active amp model in them, a single DT-50 will see both amps in Channel A.

If you turn off or remove Amp A in your HD500 signal chain then the DT-50 will switch to Channel B with a single amp.

As long as your dual amp setups don't involve turning off or toggling between the two amp models, then a single DT-50 will manage them both through Channel A.  Cross fading between two active amp models on the HD500 should also see them being heard through Channel A on the amp.

Setting the HD500 to send to Channel A for both amps doesn't make any difference with a single DT-50.  If Amp A is switched off the amp will switch to Channel B.

You can set both amp models to PRE, both to FULL or one to PRE and the other to FULL.  The output will be summed to a mono signal with a single DT-50 amp.

Your question made me go and get the DT-50 out to check my answer before I posted my reply.

HTH

Nick