You may need to upgrade your AV amp in order to use external power amp.
Alternatively, you can modify your AV amp to get the pre-out function, but it may be better for you to sell your entry level av amp and buy a medium level with pre-out.
As AV amp's functionality / standard will be changed in every year (DTS, DTS-ES, Dobly Digital, DTS-HD, Dolby Digital HD etc...) , so we recommend to buy a entry / medium grade AV amp with pre-out function and use the UCD as power amp.