Well....

While the 533-pt's and the in ceiling speakers are rated at exactly the same sensitivity they are very different beasts indeed.

Even without any insertions in the signal path I would expect louder volume from the 533-PT's.

The 7075 is only driving the midrange and tweeter in the powered towers with the built in 150 watt amp driving the 8" woofer. depending on where the crossover is (and with an 8" driver it could be fairly high into the low midrange) a significant portion of the signal could be handled by the 533's internal amp leaving much more headroom for the upper mids and highs.

The ceiling speakers have to reproduce the full range signal (as much of it as they can) using the 7075 alone.

Also, do you have an SPL meter? This might be a good time to break it out. If you really want to know what the relative volume is for a specific level setting on the 970, SPL readings taken from similar distances will at least give you a more accurate comparison than your ears and brain will.

One last thing, keep in mind that as passive devices, the Niles volume controls CUT volume only. When the volume control in the kitchen is set to full volume, it's simply at the setting where it is reducing the signal least.

Having said all that, there could still be an issue somewhere in the signal path. Why not do a bit more troubleshooting and post your results?

More information will help to troubleshoot your issue.