OK, your preamp does not have a Home Theater mode, so you will have to remember what volume setting you use.
First, make sure all your equipment is turned off. Make sure the volume controls on the preamp and receiver are both turned all the way down before you power down.
Connect the RCA unbalanced input on your preamp to the Left and Right Main channel RCA outputs of your MRX710. For the purpose of this example I will just say it is the TAPE input.
Connect your Preamp to your BHKs with either RCA or XLR cables.
Connect your BHKs to your speakers via speaker wire.
Turn on your equipment, check for ground hum with the volume controls all the way down.
You can and will use the room correction on your Receiver to your advantage for this next part. If you have a sound level meter your can use that to set the initial volumes.
Using a source with constant level, like a test tone through all your channels, set the volume control on your Receiver to a low to moderate level. Now slowly turn up the volume on your preamp until the volume of the main speakers closely matches the other channels. Make a note as to the volume level on your preamp (you want it to be an easily remembered and number that you can repeatedly set when you want to listen to multi channel.) You can play with the 6 db correction if you want, but it is not necessary. If you end up with, for example 33 as the setting, I would choose to put the volume on 30 or 35 so you can remember and easily reference it. You do not want it to be a significantly higher level than a safe listening volume ( for example 70 to 80.)
Next run the room correction process on your receiver, and it should be able for any small difference in output to the front left and right speakers. You might want to run it more than once, and play with different settings to your preamp volume control to see what works best. The less correction your receiver has to make on the left and right front channels the better.
You might also want to run it on a couple of different listening volumes to see how well the receiver/preamp combination volume tracks the front two channels compared to the center and surrounds. Your center channel amplifier will be your weakest link. You might consider getting an M700 to run you center speaker. The amplifiers in your Receiver will be able to run a little stronger since you will no longer be using the left and right channels, reducing the drain on the internal power supply.
Now you will only have to turn on your Receiver when you listen to multi channel, unless you are feeding a particular source through it and not directly into your preamp. When you want to listen to multi channel, you will set your preamp volume on the number the process I just described reveals (you see why I said make it easy?) An d set the input to TAPE, or which ever you end up using. You will then use the volume control on your receiver to set the system level, and make adjustments for the source material.
Enjoy and let me know how it works out.