Yes, you could power a sub with one channel of a multi-channel amp. However, most of today's subwoofers have a built-in amp to power the sub, so that wouldn't be necessary in most cases.
Also, most recent sub's also have a crossover built in as well. If you get a powered sub with both a speaker level and LFE (low frequency effects) then you'd have a very flexible sub that could work well in both a stereo or theater system; and would also allow for a variety of connection options. I.E., using the speaker level inputs (and crossover) for music, and the low-level input from the LFE or "sub" output of the receiver or pre/pro for movies.
I'm not aware of any stereo pre/pro's. Those that I am aware of are multi-channel units designed for 5.1, 6.1, 7.1, and in some rare cases even more channels.
There are many ways to start off with a relatively simple 2-channel system and grow into a full surround system. Here's a couple of examples: 1) you could purchase a decent receiver from Denon, Marantz, Yamaha, Arcam, etc. to begin with, and then add a 2-channel outboard amp for the main left and right speakers to add more dynamic impact, clarity and "grunt", and finally add a 5-channel amp later for a full 7.1 system; or 2) you could also get a pre/pro such as the Outlaw 990, B&K Reference 50, or Parasound 7100 and a 2-channel amp now, and then add a 5-channel amp and additional speakers in the future. In either case you could hook up the sub and use it now along with the stereo output, and later with the full surround setup.
