RE: single vs. dual conversion receivers
If I understand correctly, modern high-end single conversion receivers, like the FMA M5 and Berg 7P accomplish the function of the second IF through digital signal processing. That is, the analog IF signal is converted to a digital signal, then processed using the receiver's internal microprocessor to provide the filtering and image rejection functions that would have been accomlished in a analog second IF stage. In this way these receivers achieve the same flying performance capability as a "true" (hardware implemented, not software) dual conversion receiver.
Edited for clarity.