I don't wish to appear rude, but you're kind of missing the point (OP, not you Prof).
It's like looking for a car. If you have 4 kids and a dog, you don't buy an MX5. If you're young, free & single, you don't buy a Chrysler Grand Voyager.
It's the same with speakers. If you have the space for full size speakers & stands, then great, buy the 685 HT and know you've got a great speaker package.
If, however, you don't have room for full size speakers all around your room, then you've automatically eliminated the 685 HT, and the MT-30 comes into its own.
There are another couple of options though. You could get the 685s for the front stereo pair and the M1 sats from the MT-30 for the centre and surround, or even better, the 685s and the 'proper' centre from the 600 range (can't remember the model) but use the M1s sats for the surround channels (I believe the latter is the exact system Prof Hat uses).
Whichever way you go, you'll have great sound. I can vouch for the MT-30s myself, awesome sound. The PV1 is truly beauty and the beast!