The Outlaw M8 would sound pretty good. The problem is integrating the sub. If you buy a sub without a line out filter like the SB-1000 has, the typical configuration for you is to set the sub low pass crossover dial to the low frequency roll off of the speakers so that it fills in where the speakers stop producing much bass. Most home audio setups use AVRs with bass management that allows for choosing a higher crossover point because the AVR will run a high pass filter on the signal sent to the speakers. I would be surprised if your Oppo has bass management built in (it might). And if it's an LFE out, it might just be the LFE signal, which is the .1 in 5.1 multichannel audio. The subwoofer out on an AVR includes the .1 channel and whatever bass is sent to the sub from the other speakers due to the crossover settings in the bass management.
So do you want a sub just to add onto the bottom end? Or would you like to improve on some of the midbass that the Airmotiv is producing? I thought the midbass on the Airmotivs was a touch muddy, compared to what I'm used to (not bad for a little 4", though).
The SB-1000 is not really bigger than the M8. It's an ultra compact 12" sealed sub, a 13" cube. But you will feel either sub some if you turn it up and your room is not too large, with the SB-1000 with the potential to get louder without losing composure. And the SB-1000 is just a better SQ sub.
As for blu-rays, a sub would make a difference.
As for pre-amps, I use an Schiit Asgard 2 headphone amp that has pre-amp outs. Awesome headphone amp, and works great as a pre-amp, too Edited by cel4145 - 4/8/14 at 8:46pm