Ok, here's the deal;
The Lynx [when outfitted with the LT-FW card] will give you a "control panel" to allow the "hardware monitoring" capability, which basically spits the inputs [AD] right back through the outputs [DA] pre-software, as the software is really what induces the latency of Round Trip Audio [AD/DA through the input channel]
FW is an extremely limited format, as there is something like 4 ms of latency inherent in the technology, but it is workable, if not, I wouldn't have it in my project studio. Sure there is some residual latency with converter, {both Apogee and Lynx are FAST converters in this regard] due to DSP Nyquist filtering and such, but its SO residual you might never experience it.
The Driver, the Software, and the Computers speed, all deal with this issue of latency, and its all different, so its not easy to find the perfect marriage for Software and Hardware with you're chosen setup, but companies are trying: EXAMPLE, Apple/Logic and Apogee Symphony. Symphony is optimized for Logic so as to get the LOWEST THROUGHPUT latency available using PCI-e [which is DOUBLE the bandwidth of PCI and almost QUADRUPLE the amount of bandwidth in FW!] I still think either Lynx System or Apogee X-FW system is a professional means that will do the same thing respectively [with Hardware monitoring]
The Apogee PC FW drivers are horrid [unless you have the right FW chip set noted in the manual], I recommend against going Apogee FW [unless your on a Mac because the drivers actually work when you need them to]
With the FW system and AD16x, you'd only get 8 channels of hardware monitoring as the Apogee "fire-mix" software is only capable of monitoring 8 input channels, [kind of built for the Rosetta 800] but you can create as many sub mixes of those eight channels to any pair of 8 channels. Kind of limited but works never the less.
I think the BEST option with a PC/Nuendo rig would be the Lynx AES16 PCI card with either Lynx Aurora 16 and two Lynx 1605 AES cables, or the AD/DA16x system with Lynx 1603 cables. You'll be able to do hardware monitoring with the AES16 as well.