Specs on the M70 say f3 of 40hz, so you want an 1/2 to an octave above so 60 or 80. The small speaker setting is correct as that directs the lower frequency to the subs. You could also try lfe plus main as the 70s probably can keep up with your current subs, but not when you upgrade.
As for subs I'd look at HSU, SVS, and rythmik. Even at a good price of Ebay, the 505s wont do much for HT, but would likely be fine for music only.