Adding a subwoofer to a 2 channel amp

Use a powered sub and the amp won't matter. The preamp just needs another set of outputs for the sub.

Another option would be a Rel sub using the high level inputs which gets connected to the speaker posts on the back of the amp if there is no sub out on the pre

Adding a powered subwoofer is a very good idea. Easy to connect as mentioned in the above 2 posts. If you connect as mentioned by RWwear, I would use subwoofer interconnects which will render a better low frequency signal than regular analogue interconnects. They will also reduce any hum. I recommend audioquest SubX, not too expensive and also is composed with silver wiring, which is a very good conductor for subwoofers.

Powered sub, yep.
#1
Use the speaker level connections? Also yep.
Now MY question! Do you want to run the mains full range?
If yes, you don't have to daisy chain the mains thru the sub.
Just run another set of wires TO the speaker inputs on the sub. The speaker inputs on the sub have pretty high input impedance so no loading or other effects. Kind of like bi-wiring!
You probably don't need any exotic or expensive cabling. After all, you are interested in 80hz and down, only.
#2
use a splitter between pre and main. Run good quality, well shielded cabling to the subs low level inputs. Done.
The hazard with this? It is possible that the 2 input impedi in parallel will not be to the pre-amps liking. (Pre would like high impedance to work into)

Get a Paradigm X-30 external cross-over (about $100=/- used here on Agon)

I second the REL, but set the crossover as low as you can and work up. I have my Stadiums set at 22 Hz. The REL provides infra bass, which greatly enhances the realism of your system. It is not a subistute for bad or inadequate bass above 50 Hz.

I would recommend the external crossover (such as Marchand or DbX) and then driving the subwoofer with a separate amp. I have done bi-amping with a second pre-amp out and no cross-over but when the cross-over was introduced there were significant improvements. Building your own subwoofer can result in both significant savings and sound improvement to your system. Many subwoofer amps have built in cross-overs. Check out Parts Express and/or Madisound for possible places to start. Good listening!

I use the RC-972 pre-amp with a Carver M1.0t Main amp.
The speakers are Maneplaners, MMGs, coupled with an HSU research powered sub. The sub has a built in adjustable crossover, phasing switch and volume control. The RC-972 pre-amp has two outputs. I just hook the main to one set and the sub to the other. The system is easy to calibrate and sounds excellant.

I, too have Maggie/HSU setup. Absolutely seamless with very musical bass. My Listening room is about 4000ft3 and the VTF2mk3 fills the bill.
Not bad for flicks, either.