7.1 is IMHO not too likely to take off as anything more than a niche in a niche. Look how hard it is getting people to go for 5.1!
Having just said that does not mean I think it is a bad thing though. Quite the reverse - depending on the configuration you are using (there are a scary number of 7.1 combinations once you factor in height channels for stuff like BD).
I can definitely go for the additional side channels, and if you're doing film work it would be a good thing to be capable of creating SDDS type mixes (with 5 channels across the front) as these are used in some cinemas.
6.1 - I have a 6.1 setup, although this was mainly because I had to buy the speakers in pairs. My main studio setup is still 5.1, but will (probably) go to 7.1 this year.
6.1 does seem to be gimmicky, but there is content for it. I can think of several DTS-CD that have DTS-ES 6.1 discrete mixes, and the "Lord Of The Rings" films are also in 6.1 DTS-ES.
Which brings me to what may be the biggest problem of all with 7.1 - what do you write it to or play it on?
AFAIK, there are only a very few encoders that can create 7.1
DTS-HD MAS will create you DTS Master Audio files in 7.1 that can be played straight off a suitably equipped PC with the DTS-HD StreamPlayer (also handles all other DTS types).
I also know that Dolby True HD will create 7.1 streams, although with Dolby True HD you must be very careful - not all streams that claim to be 5.1 or greater in this format are actually in Lossless - in all Dolby True HD streams there is a "hidden" data-reduced Dolby Digital version too, in case your Blu7 Ray player is one of the vast majority that cannot actually decode True HD in anything greater than the mandated stereo - Dolby True HD is only optional at anything over 2.0.
Basically, for 7.1 you have Blu Ray, and Blu Ray only for the time being. And not the cheap Abstraction Layer types either - these will only create BD-R and BD-RE discs, cannot replicate and to get content onto a disc you must use either Sony's Blu Print or Sonic Scenarist BD edition. I do not know of any of the cheap (as in under $0,000's of worth) apps that can actually use DTS-HD MAS - Adobe's Encore DVD for example will read the file - but when it imports, it stretches out the length as if it were a stock "legacy" DTS stream at 1536kbps.
6.1 is easier to get onto disc.
You can do this to straight CD - using DTS-ES-WAV forms, which will take the 6.1 mix (discrete or matrixed) and fudge the header to make it seem as if it is a straight 2.0 16/44.1 WAV file. When played back through DTS-ES systems, you get the fully discrete mix out.
DTS-ES can also go onto DVD-Video, and if the Video_TS is correctly authored, can also go to DVD-Audio.
One more small comment.
I am not at all familiar with the Yamaha amplifier mentioned.
Can you bypass all tone controls?
Is it Class A circuitry, or is this a consumer "reciever" type?
I ask as a HiFi amplifier is inherently unsuitable for 5.1 monitoring. They all colour the output in some manner. WHat I would highly recommend for surround work of a commercial nature is a good set of Active Monitors such as the Adam A7. This way, you are using matched amps to the drivers, and the mixes will translate better (assuming correct calibration both electrically & acoustically)