Is it me or will Jcbtubes' solution not work. Please forgive all inaccurate units of measure in the below example, it's just to explain with.
Say the 9 oclock position of the preamp amplifies whatever comes into it by 2. Say you level it at that position, and say the volume of the HT receiver/processor is at a volume setting that puts 2 volts to your preamp. So this would be 4 Volts out of your preamp, and whatever speaker volume this generates is matched to the other speakers (being powered presumably by the receiver).
But now you turn up the volume level of your receiver, which now gives 2.5 volts to the pre, resulting in 5V output. There is no guaranteeing that the output of your 2 main speakers when given 5 volts will still match the other speakers' output when given their dose of this new volume setting from the receiver. You would have to level the sound across several volume settings to make sure it happened to work out, and I doubt it would. I think this is the same type of math that makes a 40 year old exactly twice as old as a 20 year old, but only for that year. You don't know what those extra years will do to the geometric ratio.
I would recommend finding out what unity gain is on the pre, and always setting it (and leveling it) to that when you use your receiver. This way your voltages are always being multiplied by 1, and therefore will always linearly match. Either that or just use the HT Passthru input on the back of your pre ;-)
I'm not 100% confident in this theory, someone please call BS if it is.