Hi Grant. An older amp may not have all the same types of audio inputs to match all the various types of audio outputs on a new digital pre/pro; however, any amp regardless of age should have RCA type analog audio inputs that you can connect to the RCA type analog audio outputs on the digital pre/pro. Of course, if the amp is only a 2-channnel amp then with multichannel pre/pros such as those you mention you would only be using two of the eight available RCA analog audio outs on the pre/pro.
As for the brands you mention (Nuforce and Emotiva), I have no personal experience with either but both are reputable and have plenty of fans. NAD is another brand with a good reputation for value. The Emotiva XMC-1 ($2000) looks like it will have substantially more features than the Nuforce AVP-18 ($1100); however, be aware that Emotiva has been talking about bringing the XMC-1 to market for quite some time now (with major design changes along the way) so who knows how long you may have to wait for it. On the other hand, a concern I would have about the AVP-18 is that nowhere does Nuforce indicate what DAC chip it uses. This makes me suspicious as to its quality. Also, a noteworthy feature advantage of the Emotiva over the Nuforce is that the Emotiva will have streaming audio capability whereas the Nuforce appears not to. Furthermore, the AVP-18 has a single USB port which is intended for firmware upgrades, not audio. In contrast, the XMC-1 has three USB ports designed with audio in mind, including support for digital audio file formats (FLAC, WAV, AAC, and MP3) which the Nuforce appears to lack. Of course, how important these differences are will be influenced by what other equipment you might be using.
When you ask will I be losing any quality or limiting my options if I go with these choices (referring to the AVP-18 or the XMC-1), thats impossible to answer without knowing more about exactly what kinds of source material (audio CD, SACD, DVD, digital audio files) you want to accommodate, which components you already have and plan to continue to use, which components you plan to buy, and your budget.